coming up with accurate number regarding judgement

[Mikey]

Lets say I think my opponnent will fold 10% of the time if I raise or he'll fold 15% of the time if I raise or 20% of the time if I raise, how can I come up with a number like this accurately??

Can anyone give me an example of how they come gauge whether or not if they raise, there opp will fold X % of the time or not?? How can I get real close to this number?