Keep in mind that all wheels aren't created equal. The mounting plane from the outside edge of the wheel will vary between wheel styles - even within the same manufacturer. Easiest way to do this is to measure the front of the chassis and find the centerline, then measure the rear of the main chassis plate and find the centerline. with the rear pod attached, place your straight edge (or ruler) at the center of the front and center of the rear main plate and mark this on the rear pod. Now measure from this reference mark. This is because the rear pods are all offset for the gearing. I have my rear track width set at max, and have to adjust the shimming for each different style rim.
good luck.