trying to simply get it to rotate once every minute. It does fairly well, but after a while it gets a little too fast. After only a few rotations actually. This stepper has 64 steps and is geared down to 1/64 so it should have 4096 steps.
So you want to have 4096 steps in 60 seconds.
That is 68.26666. steps per second.
That is a step every 14.6484375143 milli seconds
So getting your step value directly from the millis() timer is not going to work.
You need to work out a fractional division, that is one where you divide by one value for n1 times and divide by another value for n2 times. In that way you average the stepping rate you want but it jitters a bit.