Looking at the diagram provided by Curt Manufacturing, I assume they are trying to make the maximum moment applied to the receiver by the carrier match that applied by a trailer at the maximum tongue weight. If they assume the pivot point is the furthest point forward on the receiver shown, it could certainly be 2 ft in front of the hitch ball location. A trailer with a tongue weight of 600 lbs. would then generate a moment of 1,200 lbf.ft. (600x2). If the carrier moves the load 2 ft further back, then the 300 lbs. limit would generate the same moment of 1,200 lbf.ft. (300x4).
In reality, I suspect they are building in some margin as the center of mass of the loaded carrier is unlikely to be at the very back. However, I'm not certain how realistic the assumed 2 ft length of the receiver assembly is when you get away from a frame mounted receiver on a full size pickup. The factory hitch on the Ascent is a very different assembly, so I doubt that using half the tongue weight for the maximum carrier load will result in the same twisting moment on the receiver tube.
Obviously this is all based on static conditions, as driving with either a trailer or carrier will result in higher dynamic loads on the receiver when going over bumps, etc. In short, I don't believe there is really a simple formula that will define the maximum load of the carrier for all vehicle applications. The original Subaru factory ball mount places the ball very close to the receiver, which I suspect is intentional to limit the twisting moment on the receiver. Consequently, I would suspect the maximum allowable load to be well under 250 lbs (500/2) if it is placed 2 ft further back.