[Fr. Frank]

With a tandem trailer, the height of the top of the trailer frame needs to be measured between the axles as the frame is level. The top of the hitch ball on the tow vehicle should be the same height as the top of the frame height as measured between the axles. Lift the tongue too high and so transferring weight aft, and you get sway and greater braking difficulties. Drag the tongue too low, and you get asymmetrical tire wear, and tongue hop under acceleration and during braking. Both too high and too low will cost you in fuel economy.
Tongue weight on a tandem-axle trailer is not as critical as with a single axle, but should be between 4% and 7% of the total towed-vehicle weight. I generally shoot for 5%. Too little tongue weight is actually harder on your engine and tranny than too much tongue weight, because of the greater moment of inertia shift under acceleration.