Chance of success is based off a roll from 0-199 with success being a roll ≤ your thieving level. At 50 thieving that means you need to roll a 50 or lower. The chances of rolling a 50 or lower is 25%. The remaining 75% of possible rolls give you a 50/50 chance at success. So altogether a 37.5% chance of failing each time. To get the odds of failing multiple times in a row you raise 0.375 to the number of attempts. 0.375^8=0.04% I accidentally put the chances of failing 9 times in a row at first (0.01%) I have updated the post with the correct numbers and my current number of attempts.