PadX vs Cockpit

Dear FIST Fitters:

When and how do you address aerobar extension length? If I may introduce a new term to describe, CockpitX=Horizontal distance BB to shifter pivot. Aerobar extension length = CockpitX - PadX.

I tend to think of cockpit length(alternativly CockpitX) as the primary fit output and PadX secondarily. How do your think of this relationship and PadX vs HX?

Thanks in advance for your thoughts!