right then... since he is saving his metamagic school focus 3/day uses for twinning, and since he probably lacks feats to make admixture "easy metamagic". Then he would probably have to settle for adding only one admixture at a +3... making it a 540 damage fireball (half fire, half admixture damage) @ level 8 slot... 3 times per day...
He could then use lower level slots of less beefed up versions (aka, ones without the maximized and without the metamagic school focus)...
So, that means no admixture, so... spell level 6 for 45d6 fireball.