Wheelbase and weight distributing hitches

I have never heard of using wheel base as any factor in setting up a weight distributing system. Weight is what they deal with and is pretty much solely the criteria for setting up the system. Weight relates to the attitude of the tow vehicle when hitched up with trailer and truck EVEN (not necessarily level). Different manufacturers have differing instructions about how the hitched truck stance should be when correctly set up. If you don't have an owners manual for your tow vehicle see your manufacturer for instructions for set up.
Good luck / Skip