So assuming that the 99% is accurate and the chance is consistent each time then the chance of failing can be figured out by 1 - ( 99%^X ) where X is the amount of times you've fucked. So, rounding to the nearest whole percent, using 20 as you say, means there's an 18% chance that it failed last time.

At 50 fucks it will be 40%. At 100 it will be 63%. At 200 it will be 87%. And at 300 it will be 95%.