If I'm envisioning this right, you are shooting with your right shoulder, and want to drop the left shoulder?
Technically, at least for me, both of my shoulders are relaxed (I guess what you are referring to as dropped?) while laying straight behind the rifle and having my elbows/arms be square/perpendicular to the rifle. If you are having to drop one shoulder that means your other shoulder isn't relaxed and you are probably not loading it correctly when getting behind the gun and shouldering it.
The proper way to shoulder the rifle would be to lift the butt up a bit while in position and lower it into your shoulder (not bringing your shoulder to the rifle) and kind of pushing it downward/inward in an action thats similar to how a break-open shotgun closes.