It might be a good idea to find out the same pain level for slaps on the balls.
Start softly, remember balls trapped in rope are much more vulnerable
try making each hit a little harder.
Does your slave find your feets sexy, if so try kicking his balls. Remember careful, you don't want to break your new toy. As Long as He is hard he enjoys it, even if he says otherwise.
continue hitting or kicking until he is no longer hard, so you know hom much pain he likes.