Easy formula, let's say you're 3:1, you've got 25% chance to hit.

X:Y as percentage is Y / (X+Y)
3:1 as percentage is 1 / (3 + 1) which is 1/4 which is 25%

For stranger ones, you'll just have to guesstimate, e.g.

7:2 is 2/9 which is ~22%

Wow, my first 2+2 posting... you might as well back up the money truck now