Ford 2003 Mustang Workshop Manual page 2641

Pull the tape measure up to the steering wheel. Position it
through the steering wheel opening.
Depress the clutch fully and start the vehicle.
Shift the transmission into reverse gear, then move the shift
lever forward to the position in between reverse gear and
neutral. Slowly release the clutch pedal (if the vehicle creeps
rearward, move the shift lever forward a little more). The
transmission will behave as if in neutral, but the reverse gear
synchronizer will be bypassed.
With the clutch pedal up, gently pull the shift lever toward the
reverse gear position. Gear clash can now be heard.
Slowly depress the clutch pedal. Using the steering wheel
inner seam as a reference, note the measurement when the
gear clash stops or the shift lever slides into reverse gear.
Depress the clutch pedal to the downstop. The difference
between the measurement taken when the gear clash