Not quite sure what Mikumaru is saying, but magic damage+ is added to the D of a spell, where D = V + (dINT*M). Each spell is assigned a value of V and a value of M (which also depends on dINT range), so how much this improves your damage will vary greatly depending on spell, target, and your equipment.
Did some testing on SCH/RDM against Blanched Mandragoras:
Set 1:
Soothsayer MAB R15
Bokwus Circlet
Bokwus Robe
Yaoyotl Gloves
Icesoul Ring
Strendu Ring
Novio Earring
Strophadic Earring
Eddy Necklace
Bokwus Slops
Bokwus Boots
Mephitis Grip
Merciful Cape
All Bokwus r15 MAB. Char is a taru with +7 int merits.
Set 2: same as 1, except for Buremte Hat.
Set 1 vs set 2 stone spells:
S1 814 861
S2 1427 1446
S3 2067 2056
S4 2968 2915
S5 4000 3497
Set 1 vs set 2 thunder spells:
T1 880 923
T2 1493 1508
T3 2169 2151
As a rule of thumb for macroing, Buremte beats Bokwus Circlet for tier 1 and 2 nukes. For tier 3, they're about even. Bokwus comes out ahead for tier 4 and 5 nukes.
its explained pretty clearly in the comment but i'll make it more clear even tho its moot at this moment . Elemental Staves , crafted and trial , are given affinity/DMG+ values ,where the first 1 point is 10% and each additional point is another 5% damage . So Affinity DMG+2 is 15% increase , where +6 is 30% . While these new items dont have the exact same description , i was interested to know if the DMG+ mechanic worked in the same manor , which from your voodoo math it obviously doesnt .
according to the affinity math , the Magic DMG+ is about a 180% boost presuming magic damage+ doesnt cap and is multiplicative not additive . +215% damage (this + a ToM Staff) sounds a little too good to be true , but if it is , I would love to see what a bunch of BLMs timing Meteor can do .