I just posed ths question in another step.
Put some good test wheels and straight axles in your car (not a bent axle used for steer) and do a slow roll test forwards and back with ~3oz of weight sitting on top of the rear. The rear wheels should migrate to the axle heads. Every time you do this, you are checking the integrity of your car. Now install the DFW (bent) axle on either side, set the steer to 4" over 4', and do a slow roll forward with ~ 3oz placed on top of the rear of the car. Watch the rear and how it tracks reference the rail. Switch the side of the DFW, set the steer to 4" over 4', and again do a slow roll forward. Watch the rear. One side may be favored with the rear tracking more towards the center. At this time, you may or may not have to narrow the FDW, depending on how the rear is tracking, but you want to keep the rear centered to whatever extent possible. The cant of the FDW alone may be enough to keep it off the rail.