Ultima does not use level at all. (a.mat * 7.5 - b.mdf * 2) + 11000
Bahamut is (15000 + a.mat * 10) + (a.level * 10) - b.mdf * 2, so at max level contributes 990 damage.
I had toyed around with a bigger weight on level for summons. The black magic seemed fine all game, so I didn't really touch it.