Raise the whole gun. I have 12-18 inches, depending on the drop, under the forearm. That will allow you to sit up in an upright geometry, as if you were standing, keeping the butt plate off the boney part of your shoulder and also your face does not creep forward. You may need sand bags of 6-8" under the butt also.

I use a lead sled on occasion, but for really hard kicking setups like 3.5" magnum slugs from a 6.5 pound shotgun. Have not tried it with ML's yet.