I originally posted this in the ASA thread. At the time we (my BLM partner), wasn't sure what they did and we wanted to find out. Some people were skeptical, assuming it was +10% more crit damage, or 10 MAB to 11 MAB on crits (10% more MAB).
So for those that are wondering, we finally got off our butts and decided to crunch some numbers. The results of our findings show that "Magic Critical Hit Damage +XX%" gear adds an additional XX MAB during Crits. This does not include the +10 MAB you already get from a critical hit. If you have Magic Critical Hit Damage +10%, you will have 20 MAB (+10 for critical, +10 for gear) during crits.
We performed our tests on Wild Rabbits in Ronfaure, which have a base INT of 6. All tests were done on non-corresponding days/weather. No MB modifiers.
BLM was geared as follows:
HQ Staff
Wise Strap (Crit Rate+3%)
AMK Hat (INT+4 Crit Rate+10%)
Witch Sash (INT+5, Crit Rate+2%)
ASA Pants (INT+2, Crit Dmg +10%)
Phantom Tathlum (INT+2) was used as a control (pants).
Including his merits and gear, this gave him 92 INT and 42 MAB (LTNG) or 97 INT and 42 MAB (ICE).
Started out with LTNG II tests:
Non-Crit: 430
Crit w/o pants: 460
Crit w/ pants: 490
Burst II test:
Non-Crit: 1439
Crit w/o pants: 1541
Crit w/ pants: 1642
Freeze II test:
Non-Crit: 1455
Crit w/o pants: 1558
Crit w/ pants: 1660
During our testing, we would compare all our numbers against the known magic damage formulas posted on ffxi wiki. In all of our tests there was no discrepancies in the Non-Crit and Crit w/o pants.
After that, we just assumed an additional 10MAB on Crit w/ pants, and the numbers worked themselves out perfectly.
As for an explanation as to why "+10%". The only reason I can come up with, is that when MAB is applied to the damage formula, its done in XX/100. An additional 10 MAB would then be 10/100, or 10%.