I made a formula for the chances of success. It does not have any logic for natural 1s or natural 20s, just the chance of hitting.
1d20 = (21-n)/20
2d20 = 1-((n-1)/20)^2
(n is the roll needed to hit)
Here is the table of odds below:
ToHit 1d20 2d20
1 100.0% 100.00%
2...