Depending on how much the truck will settle when the horses are loaded, will depend on how high your hitch should be when it's empty.  We run our trailers a little nose high, but with almost equal weights on both trailer axles.

Torsion axles are more sensitive to improper pitch loading than a leaf spring suspension. The more level the torsion axles are, the more equal the weight each one will carry.

With the trailer sitting level or slightly nose high, measure the trailer hitch's heigth. When it's loaded, take a second measurement. The difference is how high the hitch should be raised, to effect a level loaded trailer.