My suggestion is to tie the Tomestone Rewards with Completion Percentage of mobs killed.

The Dungeon could track the PT's progress with an on-screen percentage and the final reward is scaled to 25%, 50%, 75%, with 85-90% equaling 100% reward but add an extra Bonus for 100% Completion (all mobs killed).

I also think they should put a few tomes in the chests too because I like opening them all and hate it when the PT just avoids those rooms/areas.