Do you have the scope base that you plan to use on the rifle? If so, attach the base to the rifle. Then cut some small blocks of wood the same height as the ring height that think you want to use. Set the blocks of wood on the base and then set the scope on top of the blocks. Cut/sand the blocks until you get the desired clearance from the front bell of the scope to the barrel. Measure the block of wood height and look at ring specs until you find what you want.