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First off these are NOT my formulas. My official stance on magic accuracy testing has always been that it's waaaaaay too much trouble for me to test, especially since I'm a non-caster. Still, I stumbled upon this on FFXIClopedia, and they seem reasonable, and apparently well backed-up by a lot of data some JP player collected. So here's the formulas in a nutshell:
(Total) Magic Accuracy = Spell's Accuracy + Magic Skill + [Stat Difference Contribution] + Gear/Merits
Stat Difference Contribution =
Your INT - Enemy INT if Difference <= 10
10 + (Your INT - Enemy INT)/2 if Difference > 10
Pretty straightforward. Basically, if the difference between your INT and the mob's INT is over 10 (which is pretty much always the case for any dedicated casting job in EXP, except possibly for Elvaan non-BLMs) every 2 INT is +1 Magic Accuracy. Just replace INT with MND in the case of the elusive WHM nuker.
NQ Staves = +20 Magic Acc
HQ Staves = +30 Magic Acc
Magic Accuracy -> Magic Hit Rate relationship
Magic Hit Rate will be defined as the probability of landing your spell unresisted. The odds of partial resists are based on that, and it's also pretty analogous to successfully landing a melee hit without it being stopped by Evasion, Parry, Shield, Block, etc.
If your Magic Hit Rate < 50%, +1 Magic Accuracy = +0.5% Hit Rate
If your Magic Hit Rate >= 50%, +1 Magic Accuracy = +1% Hit Rate
E.g. NQ Staves give +10% chance to land spells unresisted if your Magic Hit Rate is less than 50% (assuming that 10% doesn't bring you over 50%) and +20% chance to land the spell if your Hit Rate is over 50%. This certainly explains how I sleep DCs on PLD/RDM with Dark Staff.
Magic Hit Rate Formulas:
Hit Rate = 50 + (Magic Acc - Magic Evasion)/2 when Magic Acc < Magic Eva, until the 5% floor.
Hit Rate = 50 + (Magic Acc - Magic Evasion) when Magic Acc > Magic Eva, util the 95% ceiling.
Elemental Resistance is basically Magic Evasion, just like <Some Magic Skill> translates directly into Magic Accuracy. Additionally, each spell has a different amount of base Magic Accuracy (or the enemy has a different Evasion to different spells; numerically there's no difference, but it's easier to think of it as spells having different accuracies.) We know this intuitively from the fact that different enfeebles have different accuracies even against elementally neutral mobs. Apparently lower tier nukes are less accurate than higher tier nukes too. It's also known that your Magic Accuracy takes a hit (or the mob's Magic Evasion takes a buff) for each level the mob is higher than you; likewise, the mob gets a Magic Acc boost for each level it's higher than you, which is why we need Barspells, as strong as they are, to see any meaningful sort of resist.
Discussion/Applications?
If the formulas are valid, this puts a lot of stuff into perspective. For starters we have confirmation that 2 INT does indeed contribute 1 Magic Acc. Elemental Ninjutsu and Threnody are insanely potent: -30 Resistance from a Ninjutsu equates to +15% Magic Hit Rate when hit rate is bad, +30% when it's good, which means I was on to something when I was suggesting WAR/NINs and THF/NINs should use Hyoton before Distortion SCs. Death Blossom's -10 Magic Evasion provides the equivalent of 1/2 an NQ staff or 1/3 of an HQ staff to the whole party. Same thing for Enspell II's against their particular element.
So, who wants to check if the formulas are valid? Would be fairly straightforward - nuke something with Stone I enough times to get a reasonable number for your Magic Hit Rate (record the number of unresisted nukes), then vary your Magic Accuracy in any way and repeat against the same mob (if the mob won't last, pick another of the same kind at the same level.) Naturally RDMs are best suited to this because of their soloability. Obviously don't do this on Earthsday or Earth Weather. And obviously if you're going to use something like an EP mob don't come in with HQ Staff and +1million Elemental Magic Skill or you risk capping your hit rate.
Conclusions: Laevateinn still sucks without Aftermath. Liberator provides DRK with the equivalent of all 8 NQ staves simultaneously.
(Total) Magic Accuracy = Spell's Accuracy + Magic Skill + [Stat Difference Contribution] + Gear/Merits
Stat Difference Contribution =
Your INT - Enemy INT if Difference <= 10
10 + (Your INT - Enemy INT)/2 if Difference > 10
Pretty straightforward. Basically, if the difference between your INT and the mob's INT is over 10 (which is pretty much always the case for any dedicated casting job in EXP, except possibly for Elvaan non-BLMs) every 2 INT is +1 Magic Accuracy. Just replace INT with MND in the case of the elusive WHM nuker.
NQ Staves = +20 Magic Acc
HQ Staves = +30 Magic Acc
Magic Accuracy -> Magic Hit Rate relationship
Magic Hit Rate will be defined as the probability of landing your spell unresisted. The odds of partial resists are based on that, and it's also pretty analogous to successfully landing a melee hit without it being stopped by Evasion, Parry, Shield, Block, etc.
If your Magic Hit Rate < 50%, +1 Magic Accuracy = +0.5% Hit Rate
If your Magic Hit Rate >= 50%, +1 Magic Accuracy = +1% Hit Rate
E.g. NQ Staves give +10% chance to land spells unresisted if your Magic Hit Rate is less than 50% (assuming that 10% doesn't bring you over 50%) and +20% chance to land the spell if your Hit Rate is over 50%. This certainly explains how I sleep DCs on PLD/RDM with Dark Staff.
Magic Hit Rate Formulas:
Hit Rate = 50 + (Magic Acc - Magic Evasion)/2 when Magic Acc < Magic Eva, until the 5% floor.
Hit Rate = 50 + (Magic Acc - Magic Evasion) when Magic Acc > Magic Eva, util the 95% ceiling.
Elemental Resistance is basically Magic Evasion, just like <Some Magic Skill> translates directly into Magic Accuracy. Additionally, each spell has a different amount of base Magic Accuracy (or the enemy has a different Evasion to different spells; numerically there's no difference, but it's easier to think of it as spells having different accuracies.) We know this intuitively from the fact that different enfeebles have different accuracies even against elementally neutral mobs. Apparently lower tier nukes are less accurate than higher tier nukes too. It's also known that your Magic Accuracy takes a hit (or the mob's Magic Evasion takes a buff) for each level the mob is higher than you; likewise, the mob gets a Magic Acc boost for each level it's higher than you, which is why we need Barspells, as strong as they are, to see any meaningful sort of resist.
Discussion/Applications?
If the formulas are valid, this puts a lot of stuff into perspective. For starters we have confirmation that 2 INT does indeed contribute 1 Magic Acc. Elemental Ninjutsu and Threnody are insanely potent: -30 Resistance from a Ninjutsu equates to +15% Magic Hit Rate when hit rate is bad, +30% when it's good, which means I was on to something when I was suggesting WAR/NINs and THF/NINs should use Hyoton before Distortion SCs. Death Blossom's -10 Magic Evasion provides the equivalent of 1/2 an NQ staff or 1/3 of an HQ staff to the whole party. Same thing for Enspell II's against their particular element.
So, who wants to check if the formulas are valid? Would be fairly straightforward - nuke something with Stone I enough times to get a reasonable number for your Magic Hit Rate (record the number of unresisted nukes), then vary your Magic Accuracy in any way and repeat against the same mob (if the mob won't last, pick another of the same kind at the same level.) Naturally RDMs are best suited to this because of their soloability. Obviously don't do this on Earthsday or Earth Weather. And obviously if you're going to use something like an EP mob don't come in with HQ Staff and +1million Elemental Magic Skill or you risk capping your hit rate.
Conclusions: Laevateinn still sucks without Aftermath. Liberator provides DRK with the equivalent of all 8 NQ staves simultaneously.
Drain II has a terribly long recast, but I'm kind of stuck using DRK/WHM since I started on that and /RDM gives MAB at my level. Oh well.)
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