((Lv+2+WSC) * fTP + fINT ) * R * S * W * MDIF * MDA

Or: Why magic damage weaponskills are mediocre, and Thunder IV is not.

((Lv+2+WSC) * fTP + fINT ) * Resist * Staves * Weather * MDIF * Magic Damage Adjustment

Lv+2 = Level(75)+2, replaces D in
WSC = Modifiers, covered last post.
fTP = WS multiplier, also covered last post
fINT = INT vs INT
Resist, Staves, Weather = self evident
MDIF = MAB vs MDB
Magic Damage Adjustment = Shell, MDT gear.

The main problem with magic weaponskills is that chance of Resist right there, and the utter lack of magic accuracy gear for the NIN using Blade: Ei.

Instead of weapon DMG like in the physical equation, magic WS use the character's raw level, which works out to the advantage of one-handed weapons.

WSC is as before, complete with level penalty, though double-dip happens from INT this time around.

fINT is the difference between your INT and the target's INT. More specifically, fINT = 8+ΔINT/2 with a cap of 24, and ΔINT is your INT - target's INT. Due to it being calculated after fTP, it doesn't have as much effect as fSTR does on a similarly-modified physical WS.

(Lv+2 + WSC) * fTP + fINT = 
WD, total damage before multipliers.

Resist is magic accuracy vs magic evasion, and determines if you get full damage, or a 1/2 1/4 1/8 1/16th resist. MACC is presumably based off skill with the weapon, but no serious testing has been done.

Staves is the reason that SE will never make a weapon better than HQ staves, because they get their own special spot in the equation. Okay that's a lie, but a HQ staff matching the element of the spell (WS, in this case) gives a 1.15 bonus, and NQ gives a 1.1 bonus.

Weather is the 1/3rd chance at the bonus damage from matching weather / day. A 1.1 multiplier for matching the day or weather, 1.2 for matching both, 1.25 for matching double weather, and 1.35 for matching day and double weather. Wearing a matching elemental obi changes the 1/3rd chance to 100%.

MDIF is simply magic attack bonus divided by magic defense bonus. So a BLM75 with MAB+32 native, using Vidohunir on a RDM with MDB+12 native, would have a MDIF of 1.32/1.12 = 1.17~

Magic Damage Adjustment is for spells like Shell. For example, Shell IV reduces magic damage by 56/256 (21%~), and would thus be a magic damage adjustment of 1 - 56/256 = 0.78~

Breaking it down like before,

DMG from Level+2 
+  
More DMG from WS modifiers * level penalty
*
fTP Multiplier 
+ 
DMG from INT
=
WD


MACC vs MEVA resist penalty, if applicable
*
Elemental Staff bonus, if applicable
*
Weather / Day bonus/penalty, if applicable
*
MAB vs MDB
*
Magic Damage Taken bonus/reduction
=
Multipliers


WD * Multipliers = Damage


Important points:
fSTR and fINT are comparable as damage bonuses, but fSTR has more effect on final damage per stat point.

Lack of magic accuracy, magic attack bonus, and similar stats hurt melee jobs trying to use magic weaponskills, but lets BLM hit respectable numbers.

Lack of weapon damage (As Level+2 is used instead of DMG) actually helps magic WS, since you're not typically going to have a DMG:78 one-handed weapon.

MDIF is worlds easier to calculate than PDIF.

Thunder IV has a base damage of 541, magic weaponskills all have a base damage 78. More on that in the next post, whenever that is.