The 2006 International Building Code calls out the tolerance for how consistent each stair tread and riser needs to be in a set of stairs. The code refers to it as dimensional uniformity. What the IBC states on stair tolerance is that the difference between the largest and smallest riser height and the largest and smallest tread depth can’t be greater than 3/8″ within a flight of stairs.
With spiral stairs the 3/8″ is measured at the 12″ walk line.
