Another thing to add to formula, if you already have say a camper, golf car, load of wood in the bed, the bed is already level to a bit lower than front, the formula does take into account the amount removed from FA by bed cargo.
I took off close to 1000 lbs off the fa of my transit van last week with a 4500 lb pallet, I could not get over or in front of the RA. I have one of those with a 4' body extension on that back. So that WHOLE 4500 lbs was removing eight from the FA.
Their is more to this than a simple formula.
Marty