I've always used 18 inches to the top of the ball as a starting point, normal vehicle ride height.
For long trips (over 200 miles) I adjust the trailer load, maintaining reasonable tongue weight, and use load equalizer hitch and/ or air bags/air shocks to fine tune/level trailer, vehicle attitude and ride height / trim This can sometimes be an arduous process, but well worth it in wear and tear on my nerves/attitude on the road.
Over the years, I've acquired quite a selection of drop hitches, from 1 to 8 inch offset, remember, a drop is also a lift, if you flip it over...(Just can't pass 'em up at yard sales for a buck or two, lol)
Paul