Simpler method: Use any bathroom scale and place a 2x4 block on it (cut it about 4" square). Take a 4-foot long 2x4 and place one end on that block of wood. Place the other end on a couple of 2x4 blocks on the ground. Place your tongue jack onto the long 2x4 at the mid-point. REad the bathroom scale. Multiply that reading by 2.
Example: 150 lbs read on the scale would indicate your tongue jack is supporting 300 lbs.
If you wish a more delicate/accurate measurement, make the block on the bathroom scale and the one at the opposite end shaped like a pyramid for more accurate placement.