When I shoot off sandbags I rest my forearms on sand bags about halfway. I then hold the grip the same as if I was free standing, unsupported. Then rest the barrel just in front of the frame very lightly on a sandbag. Just enough pressure to steady the sights. Let the gun recoil like you were freestanding. I have found this to keep point of impact close to if not the same from the bench and freestanding.