Greg- we clamp the flat face of the wheel well in the car and measure over...this gives us a ruff reference when trimming the tub to fit once it is all tacked together. Speeds up the fitment process.
Can't say enough about what and the quality of work that Troy and the guys at BBT are doing to the mustang. All I can say is make sure you subscribe to the thread cause we have only scratched the surface, there is plenty more killer stuff to come!!