I was going to start with a manual table and then go from there. According to one of the developers (read about it
here) it is based on probabilities and isn't easily calculable. So I think a static table is the way to go.
I was thinking some sort of window you can type the target number and drag the result into to determine the success level. I'm not sure if this is the best way to do it but I'm thinking it should be easy enough to implement. Maybe even a simple slash command "/success (roll) (target)" and have it output in chat the success level.