I just measured my arm pad reach measurement (following Trek’s Speed Concept Fit .pdf) and came to the realization that my measurement is 49.5cm from center of pads (Zipp Vuka Pads = 11.2cm length) to center of bottom bracket. In addition, I recently tried swapping out my 100mm Zipp 145SL stem for a 110mm Zipp 145SL stem and my cockpit dimensions feel much better. This measurement correlates to my fit from a year ago which was based upon a different set of aerobars/pads measured from nose of saddle to back of pads.
So, it would appear that measuring from center of pads to center of bottom bracket to establish ones arm pad reach is more precise or constant than measuring from nose of saddle to back of pads, since items such as saddles, aerobars and pads all vary in dimensions and are not constant?