Hey Guys,
If you put the shim under the front mount, it puts additional space beween the line of sight and the path of the bullet thus moving the impact point down.
You can use metal shims made just for this, I'm sure Brownells would have some, but my gunsmith cut a piece of Oak Tag (heavy, glossy, tan colored paper grade school kids use for art projects) about 5/8 inch by 1 inch with a hole in the middle for the mounting screws to pass through. It was installed 29 years ago, and I've never had a problem to date.
El Lobo in NM