With weapon enhancement once you fail it gets reset back to +0 (or so I heard). So chance of reaching is the chance of continuously getting a success till you reach that forge level. Chance of success is literally chance of success
For example if we use a coin toss and heads and tails:
chance of reaching 1 heads: 50% (possible results - H, T)
chance of reaching 2 heads in a row: 25% (possible results - HH, TH, HT, TT)
chance of reaching 3 heads in a row: 12.5% (possible results - HHH, HHT, HTT, HTH, TTT, THH, THT, TTH)
etc.