I tend to look at as balancing the load. The eight feet on the trailer comprises 8/12 ths. of the load as long as it's laying flat. Strap it at the tailgate so that it can't shift sideways.

Remember: If you come to an abrupt stop .... that load is still traveling at 40 miles per hour and wants to shift forward. If the load is placed on a tailgate that's in the upright position .... it may pole vault over the cab of the truck.