If your Bow is low when hooked to the tow truck, set a level on the trailer frame then measure from the frake [ At the hitch ] to the ground .
Next , set your trailer jack and commence to jacking up that baby up .
When the level reads level [ or lust a tad high at the hitch ] measure again .
The difference of the two measurements are the ball height you need.
And of course this is all done on flat ground.
F350 4x4/ Dodge 2500HD 5.9L Cummins
Leaving the Picture of the Ford cuz I miss it
Pacific 2325 cc