In theory, nothing's wrong with your method. In case 1, you have the total weight of the trailer on the platform - it doesn't matter what it's resting on; jacks, wheels or anything else. In case 2, you have the axle weight of the trailer on the platform - the difference is pin weight.
In practice, for this to be absolutely accurate, in case 1 you must have no pin weight whatsoever left on the truck, but you must also not lift too high such that the pin is pulling upward on the jaws of the hitch and transferring some of the weight of the hitch/truck to the front jacks of the trailer.
Rusty
