The calibration routine calibrates what the powergear system believes as level, as in if you level in auto or manual, the green light goes on, but it is not level, then you need to calibrate.
I have always found I can level in manual much lower than the automatic.
I always dump the air to below 50 psi by holding down the dump button and stroking the brake pedal, before even powering up the leveling system.
I do not believe a box on sticks can roll. If needed, i'd rather lift the front, but do not fear lifting the rears. This box on stix is not "rolling" anywhere.