[ericand]

I think the math works out very simply if you work in radians. If you want to follow a curve of radius R through an ARC of angle A (in radians), then the inside track of the robot will travel a distance of R*A. since the outside track of the robot is at a fixed distance from the inside (by the width of the robot W), the distance traveled by the ouside would be (R+W)*A.

Distance inside of curve : Di
Distance outside of curve : Do

Di = R*A
Do = (R+W)*A = R*A + W*A = Di + W*A

You can use simiar math to compute the robot's heading change based on the distances traveld by the right and left sides of the robot and the width of the robot.

Note that by using radians you can avoid the need for trig functions.