About the percentage thing, I dont know if it applies to all snakes, makes sense if it does, but I read on this forum somewhere a guy was asking about prey size for his ball, and someone said 10-15% of their body weight is the way to go.

About the snapping, did you maybe handle some of their food products before trying to handle them? Make sure you wash your hands before you handle them as well, reason: I think along with your hands being clean, it also doesnt have a smell.