No stupid questions, right? Have you ever measured the amount of bell you put on the case mouth when you are setting up your die? I try to shoot for about .006", but sometimes go around .010" just because the dies are so touchy.
For example, with 9mm, the OD at the end of a loaded case is .380". I try to bell my cases between .386" and .390".
Or do you just do it by sight/touch/feel?