Seems to me that the working aisle is - the clear aisle distance (meaning either the distance between rack facings or pallet rack guards) or if pallets over hang the rack beam face, which is very common) the distance between pallets +the right angle stack capability of the machine w/pallet + add an extra 4" to 8" for improved productivity (meaning the operator does not have to be so precise when picking or placing a load) and other factors stated in the following. Actual machine right angle stack capability can vary from published specifications due to different floor surface concrete finishes (some floor finishes have a higher coefficient of friction than others), tire type (tire compound durometer ratings) , conditions of tires & environment (freezer, cold storage, wet floor surfaces, manufacturing tolerance (all metal machining, cutting, etc have a tolerance of + or - something as well as tire OD can vary slightly from one brand to the next - if a machine was build with everything on the + side of these tolerances (or as the design engineers say tolerance build up) it would not perform exactly as published or the engineers planned. etc). So the safe side is to add a few extra inches to allow for these factors. More than once has a customer set up his new rack system and found the truck he bought, couldn't work in his aisles- "and then the fight was on."