You use the arm-under method? I don't really like that technique, myself. It always ends in bruised shoulders, because of the lack of absorption. Not that my way is the only way to go, but I much prefer to use both hands to absorb the pressure before it hits my shoulder. I do drop my elbow, or more accurately, my forearm, as Hazim said. Just make sure you don't slam your elbow.
I can't give you some pounds per square inch in terms of how much pressure. You have to figure out what doesn't hurt. If it feels good, its right. Rolls are kind of simple, that way.