The sure way to hit the right spot is to put a pointed rod in a 4 way drill press vise and the correct size drill in the chuck. Then center the pointed rod so the drill hits it exactly in the middle. Lock the vise in that spot and place the stock exit hole on the point and start to drill at the required spot on top of the action.
You can't miss! Hope I described it right because it makes it so simple and when you do it the logic makes you go wow, why didn't I think of it.

bill