I would try to keep the scope as low as possible. With Weaver rings, you should be able to find a gun shop that will let you try low, medium, and high rings. The lower the scope, the closer you can keep the gun to your cheek, and the less "face slap" you will get when you shoot.
I don't have any hot info on 20 gauge slugs, but two slugs that I have seen most all 12 gauges shoot accurately are the Winchester Platinum Tips and the Lightfield Hybreds.
Will