Not exactly an answer to your question, but still:
Some years (~1995..1996) ago I built my own tracker as well. Was the time when the LVB trackers etc were not available. Only the expensive Kansas City tracker (expensive).
My system is still operational. Bertus, PE1KEH, wrote in PASCAL the tracking SW (input: kepler, output: sat position). I added to that the controlling of the rotors. All corrections mentioned below (overshoot etc) are done in PASCAL in the PC. That sends and integer value to the ST6 (like your PIC) which does only one thing extra: check if the integer exceeds the allowed maximum right or left position (in case max left is at 4.5V where your range maybe 0..5V).
Items to consider next to straight degrees2voltage conversion: - overshoot (when you stop, the rotor actually moved still a tiny bit) - thresholds (to avoid oscillation & to much shaking)
8 bits give enough accuracy, I used a pot.meter to convert the voltage of the rotor voltage from eg 0..20V down to 0..5V.
http://home.vianetworks.nl/users/hamoen/pa3guo/zb_rtr_ctrl.html (includes my poor PASCAL routines)
Of course it took a terrible amount of effort/time, which I had only back then, so today I would buy an LVB tracker :-)
Still - this was one of the most satisfying projects I ever did. Seeying you home-made system following the bright ISS in the darkness of the night is soooo cool !
Have fun & success
Henk, PA3GUO http://www.qsl.net/pa3guo