I found nothing to the fact about where Ford took their measurements from,

I took measurement on my RS suspension with stock OZ's and 225/40/18 tires, from the center of tread on tire to the same on other and got close to the 1514mm you state is the tracking. I would say the tracking is measured there instead of the outer edge of tire since not all Focus's use the same size tire, but they all have the same rotor face spacing.(center to center) Well all but the RS...