the best way is a combinatioon of the above, but we/you need more info. You need to know the tongue height of the trailer when loaded, and the tongue weight of the trailer. Once you have the tongue weight, add that amount in weights, bricks, peopel, whatever (right at the edge of the bed, not on the tailgate) to the back of your truck and see how much it squats. then take the measurements as described above. And yes, every trailer will be different. I'd add one inch just to cover any additional stuff you might throw in the truck. I have 3 different hitches for 3 different trailers.