Calculate It
As a rough order of magnitude you can estimate how much the weight will change when you move the axle. Divide the weight of the boat plus trailer by the distance between the hitch and the wheels. This will give you how much the weight will change on the hitch for each foot the axle is moved. If you have tandem axles, then measure from the hitch to the point midway between the wheels. It is not essential that you are dead accurate on the weight or distances. Use estimates if needed. For example, if the boat plus trailer weighs 3000 pounds and the distance between the hitch and the wheels is fifteen feet, the weight on the hitch will increase or decrease by 200 pounds for each foot moved. 3000/15 = 200. Therefore, if your boat plus trailer weighed 3000 pounds and the distance between the wheels and the hitch is 15 feet, then moving the axle aft 3” will add 50 pounds to the hitch weight. You can also estimate the change in weight on the hitch when you add weight forward or aft of the wheels. The change in weight on the hitch will be the weight of the object times the distance forward or aft of the wheels divided by the distance from the hitch to the wheels. Therefore if you are adding a 50 pound object (a motor) to the stern which is 10 feet aft of the wheels, then the weight on the hitch will change by 50 times ten divided by 15 or a 33 pound weight reduction on the hitch. These are simplifications of more complicated equations, but are accurate enough for most uses.
