Working on a defense simulator, and the biggest problem I am having is not knowing monster's to-hit. Does anyone have data (or a formula) they can share? Doesn't need to be exact, just an idea would help. Also, CR values for LE and LH would be awsome.

Preliminary data:
Level: to-hit bonus for monsters at that level
15 37.5
16 40.25
30 75

Right now it looks like a monster's to-hit is 2.5*CR, but if someone knows the formula, it will save me a lot of test work.