Materia Bonuses are stored as signed words (not sure why) and start at 0x8FEEC8 (0x4FD8C8 in the exe) and follow this pattern:
StrBonus, VitBonus, MagBonus, SprBonus (though labeled as MDef Bonus), DexBonus, LukBonus, MHPBonus (as percent), MMPBonus (as percent)
There are 17 used rows of this and four unused as listed
here. Don't modify past 0x8FF017 though. Then you'll start screwing with elements.