Best thing to do is mount your base, and mock up the scope with some washers until you get the height you want, measure the stack when your happy. The adjustable cheek pieces make it less important, but you still want about 2" center of scope to center of bore. If you buy a 40mm objective scope with a 30mm tube, the lowest rings made will still leave the scope off the barrel a good ways... Don't get hung up on low, med ,high, when ring shopping, use an actual measurement.