Moving spare tire to rear

What would be the approx. shift of weight if I move a 40 lbs spare from the front of a fiver to the rear bumper. Would the pin weight go down by approx 80 lbs? I know there could be a lot of variables but would 80 lbs a good guestimate.

It depends on the distance from the current spare location relative to the center of the axles and the new position on the bumper also relative to the axles. If you have a Sherline or other scale just move the spare and measure pin weight before and after.