I have a Wheeler Drill and Tap fixture for scope mount on loan to me and I am about to do 2 Mauser receivers with barrels off. One large ring and one small ring. Is there anything I need to pay attention to? I only have 1 shot to make it right! My main concern is the hole position, how to assure they are centered. I hope using the fixture is a guarantee, but there may be still some tricks that I need to pay attention to. Thanks.