Well it's called the bum-line for the pilot to 'sit' himself on and the teaching was to run that under the handle on the outside of the pilots door - clearly there has to be some margin for error so if the wheels are on that line there is still adequate clearance for the tips at the front.
It is also slightly more comfortable to err forwards than back, since you don't have a crewman in the door giving 'con', to be sure the tail doesn't end up too far back.