The reason the manufacturer provides the optimum height is so that when your trailer is LOADED, it will ride equally on both axles and place the correct amount of weight on the tongue.

If, when you load the truck and trailer, the trailer is not level, you will experience very poor driving and poor fuel mileage. Many times, when a BP is attached correctly to the tow vehicle, it will appear to be at a slight angle until the horse(s) are loaded. Additionally, loading the bed of the pickup - if that is the tow vehicle, with a few thousand pounds and towing the trailer with 2 horses & "stuff" - could cause the jack wheel, weight distribution hitch, and /or chains to drag and potentially CATCH on uneven ground - WORSE YET - to catch on railroad rails at a RR crossing.

The only way you will be assured of the correct height, is to load everything up, put a level on the fender of the trailer, bed/ top of tailgate of the truck, and /or A-frame of the trailer to see if the entire setup is level.

Good Luck - hope this helps.