Okay, I have a suggestion that I think will actually bring this thread on topic. Fabricate a metal disk (probably stainless or some thin metal on a transparent substrate). On this disk draw n concentric circles or tracks. In the smallest circle cut out half of the track. Place an LED on one side of the disk and a phototransistor on the other side of the disk. Align them so that during 180° of rotation the led shines through the open track to turn on the phototransistor. During the other 180° or rotation the disk will block the LED and turn off the phototransistor. On the second track, beginning at the same starting angle, divide the track into 4ths (2^-2) and cut out alternating regions. Repeat this for all n tracks such that for the n^th track, it is divided into 2^n segments with alternating segments cut out. Also, as with the first one, arrange LEDs and phototransistors aligned with each track. Mount the disk on your stepper motor. Now you can read the states of the n phototransistors as an n-bit word which will divide your 360 degree rotation into 2^n discrete steps. Now you don't need to count steps. Instead you can count overflows of the n-bit position word to keep track of complete cycles, and just read the n-bit word to know where you are in a rotation. Now you needn't care if the motor slips.

As for controlling the number of steps, with the right kind of stepper motor you can manipulate the rotation per step by incrementing the current to all four armatures. It involves using four D/A convertors, one on each armature. The resolution of the motor is then directly related to the resolution of the DACs.

Just a couple of ideas.