Put your hitch in, measure the middle of the ball (doesn't have to be exact, just within an inch or so) (lets say you have a flat drop, it would be about 18"). Then you put the trailer on to figure out your "sag". Now measure at the same place you did before, lets say it's 17". Now you know your truck sags 1"
With you knowing it sags 1", you put the trailer (off of the truck) at the level you want to tow it at, 0 angle is of course perfect. Now you measure the same place you measured from before on the trailer, say that is 13" off the ground.
To get from 18 to 13, you'll need a 5" drop. Then account for your sag inbetween there.
I'm no math matician, and I can't even spell it right, but thats what works for me
It's (static ball height)-(loaded ball height) = sag height
(sag height)-(optimum ride height)=drop hitch inches
08 Cummins Mega Cab, 7" on 37
02 Explorer Sport
08 Brute 750 on 29.5