A new, more accurate method of calibrating the stepper's initial position. Previously, I can calculate the arm's position at any position on the potentiometer. However, this requires quite a bit of measurement and interpolation and extrapolation.
Instead, I measure the potentiometer's value once, at a known angle. In this case, its 45 degrees up from horizontal. Upon startup, the arms each move until the potentiometers hit the set value, which means the arm is at (almost) exactly 45 degrees up from horizontal. Basically the potentiometers act like optical gates.
The improvement on calibration can be seen in the vertical traverses. Previously, the robot would deviate from a vertical line by around 1/3 of an inch. Now, the deviation is less than 1/32 of an inch.