I have a very shallow desk, so my keyboard and mouse are right up near the edge. I sit fairly far back, knees just about under the edge. I use a mousepad with a gel wrist rest. I end up with my right arm nearly straight, and if I'm not paying attention to it, my shoulder drops down and the entire arm hangs from it.
When I've had deeper desks, the mouse sits back a good foot from the edge and my forearm is mostly on the desk.
When I've had deeper desks, the mouse sits back a good foot from the edge and my forearm is mostly on the desk.