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
5) Day | Weather
6) Magic Burst
7) Magic Burst Bonus
8) MAB / MDB
9) Target Magic Damage Adjustment

There is reason to believe that the Magic Burst portion is calculated after Staff and prior to MAB. Further testing would be necessary to confirm the order of the Day | Weather term 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, the value of M is lessened
For dINT > 0, but less than some inflection point: D = V + (dINT * M)
For dINT > 0, but after some inflection point, I: D = V + M*I + ((dINT-I) * 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

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

Day | Weather 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 term is capped at 1.35, even in the presence of day, double weather, and Sorcerer's Tonban.

Magic Burst

MB 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)

Magic Burst Bonus

Magic Burst Bonus is calculated as:
1.0
+0.05 when Sorcerer's Gloves or Sorcerer's Gloves +1 are equipped in a magic burst.
+0.05 when Static Earring is equipped in a magic burst.
+0.03 when a level 2 Ancient Magic II spell is used in a magic burst.
+0.06 when a level 3 Ancient Magic II spell is used in a magic burst.

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.

After calculating the ratio of MAB to MDB, this term is truncated to two decimal places.

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. The base value is 1 (256/256) and is adjusted according to the fractional values in the charts below. The values are added together in the presence of more than one adjustment.
ex. Ahriman have a TMDA value of (1 - 64/256) = 192/256
ex. An Evil Weapon with Shell has a TMDA value of (1 - 32/256 - 24/256) = 200/256
Note: For player targets, this value is capped at 128/256 (50%).

Monster	Base Adjustment (%)	Adjustment (256ths)
Ahriman	75%	-64/256
Black Puddings	125%	+64/256
Cardians	75%	-64/256
Corse	75%	-64/256
Demons	75%	-64/256
Evil Weapons	87.5%	-32/256
Ghrahs	87.5%	-32/256
Magic Pots	50%	-128/256
Effect	Base Adjustment (%)	Adjustment (256ths)
Shell	90.6%	-24/256
Shell II	85.9%	-36/256
Shell III	81.2%	-48/256
Shell IV	78.1%	-56/256
Shell V (1 merit)	75.8%	-62/256
Shell V (2 merits)	75%	-64/256
Shell V (3 merits)	-	-
Bubble Curtain	50%	-128/256
Shining Ruby	96%	-10/256
Player Equipment	varies	varies

Sources

http://www.bluegartrls.com/forum/viewtopic.php?p=297899#297899
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