I may post a longer answer later but pronation/supination is about that foream only. It is the complex movement of the radius around the (not completely) still ulna it also involves the wrist.
Sucky ref but try the Nazi salute, max external rotation of the humerus and in the opposite direction max pronation of the forearm. You should feel tension. On the outer side of the forearm up into the pinky. Try keeping the scapula and try to reach/ push forward.