We have a very similar setup and have the code working excellent. Email me tomorrow at the following:
matthew.reiland@gm.com and I will send you our code for 4 wheel steering with 1 motor and 1 potentiometer.
Ever since we used the inverse SIN approximation posted here by Leo the thing has been money
We are using a 10 turn pot because of the way everything is geared and it is pretty precise but a 5 turn would be perfect. Ohh well so the robot tracks just a bit off sometime.