Richard is correct that the change in tongue weight will be neglible within reasonable chenges in ball height (+/- 2 inches).
The math works like this:
1. The weight distribution between axles will change, but the tongue weight very slightly.
2. The change in tongue weight is computed geometrically. You need to locate the center of gravity of the trailer, which is probably a foot ot two above the floor, and 2 to 4 feet ahead of the center point of the axles. You draw a line from the axle to the hitch ball, and a line perpendicular to that line that passes through the center of gravity. As you tip the line forward, by lowering the ball, the perpendicular line will also tilt forward (may 1/4"?) indicating how far the center of gravity has moved. Calculate the weight shift using basic trigonometry.