Calculating Magic Damage

Actual Damage

Actual Damage is calculated by multiplying the following factors in order and flooring after each step.
1) D
2) Multiple-Target Damage Reduction
3) resist
4) Staff Bonus
5) Day | Weather Bonus
6) Magic Burst Bonus
6a) Magic Burst Bonus Gear #1
6b) Magic Burst Bonus Gear #2
7) MAB / MDB
8) Target Magic Damage Adjustment

There is reason to believe that the magic burst portion is calculated prior to MAB. Further testing would be necessary to confirm the order of the other parts with respect to magic bursts.

Explanation of Factors

Calculation of D

Each magic tier has an inherent mutliplier value M.
Each magic spell has a base value V.
Let dINT be (Caster's INT - Target's INT).

For dINT < 0: D = V + dINT (when dINT is a penalty, the tier mult. is always 1)
For dINT > 0, but less than some inflection point: D = V + (dINT * M)
For dINT > 0, but after some inflection point: D = V + (const + (dINT-const) * M / 2))
(above some critical value, adding INT becomes half as effective)
For dINT > 0, but after some cap: D = cap

M = 1.0 for all I/II single-target nukes
M = 1.0 for all -ga spells up to Blizzaga II
M = 1.5 for all III single-target nukes
M = 1.5 for all -ga spells Thundaga II and beyond
M = 2.0 for all IV and AM I/II spells.

V values by spell:

Spell	Earth	Water	Wind	Fire	Blizzard	Thunder
I	10	16	25	35	46	60
II	78	95	113	133	155	178
III	210	236	265	295	320	345
IV	381	410	440	472	506	541
I-ga	56	74	93	120	145	172
II-ga	201	232	266	312	350	392
III-ga	434	480	527	589	642	697
AM I	577	630	552	657	526	603
AM II	710	710	710	710	710	710

Multiple-Target Damage Reduction

single-target spells = 1.0
-ga spells with one target = 1.0
-ga spells with 2 through 9 targets = 0.9 - 0.05T where T is the number of targets
-ga spells with 10 or more targets = 0.4

Resist

resist = 1, 1/2, 1/4, 1/8, or 1/16

Staff Bonus

HQ Staff matching element = 1.15
NQ Staff matching element = 1.1
No staff = 1.0
NQ Staff ascendant element = 0.9
HQ Staff ascendant element = 0.85

Day of the Week | Weather Bonus

Day | Weather Bonus is calculated as:
1.0
+0.1 for the correct day
+0.05 for Sorcerer's Tonban
+0.1 for single weather
+0.25 for double weather
This is capped at 1.35 in the presence of day, double weather, and Sorcerer's Tonban.

Magic Burst Bonus

MB bonus is 1.0 for no MB and 1.3 for a 2-WS renkei, gaining 5% per extra WS in the renkei series (i.e. 1.35, 1.4 etc)
Sorcerer's Gloves multiply the magic burst bonus by 1.05.
Static Earring multiplies the magic burst bonus by 1.05.
ex. floor(floor(floor(floor(... * MB) * Sorc.Gloves) * St.Earring) * ...) -> floor(floor(floor(floor(... * 1.3) * 1.05) * 1.05) * ...)

Magic Attack Bonus / Magic Defense Bonus

MAB is divided by MDB.

MAB is as we know it (i.e. MAB I = 1.2, MAB II = 1.24 etc). Items with the description "Magic Atk. Bonus"+X means to add X/100 to the current MAB. For example, Moldavite Earring which states "Magic Atk. Bonus"+5 means to add 0.05. Correct element potency merits are +2 MAB per merit.

MDB is 1.0 if not present and depends on the target of the magic.

Target Magic Damage Adjustment

Target Magic Damage Adjustment is as it seems. Certain targets are designed to take more or less % damage. Values for this portion of the equation appear to use the 256 number system.

Examples:
Ebony Puddings in Mount Zhayolm appear to take 25% bonus magic damage (320/256).
Eo'ghrahs in Grand Palace of Hu'Xzoi appear to take 87.5% magic damage (224/256).
Evil Weapons take 87.5% magic damage (224/256).
Demons, Ahrimans, Corses, and Cardians take 75% magic damage (192/256).
Tiamat and Jormungand take 50% magic damage.

Sources

http://ffxi.allakhazam.com/db/jobs.html?fjob=3;mid=1137029463237848499;num=50;page=1
http://ff11.s101.xrea.com/index.php?%B4%F0%C1%C3%C3%CE%BC%B1#content_1_7