It's important we are in agreement on our terms and numbers other wise....you remember what happened to that Mars rover that crashed into the surface of the Red Planet 'cuz some of the scientists were using inches and some were using cm.
Stack and Reach are measurements to the top of a head tube and useful/mandatory on a mortal bike (one with a normal stem).
Pad Stack and Pad Reach (I prefer Pad Y and Pad X to reduce confusion) are used to measure prescribe a triathlon super bike (one with an proprietary stem or stems if we're lucky).
I'd like Pad X to the center of the pad but can get really close if all you have is the measurement to the rear.
If you've had a fit I'd gladly look over the report to help guide this process
If you want to be fit I can help recommend someone who can prescribe a tri bike
If you have a current tri bike and you love the position I can explain a way to measure that PadY/X at home
If none of the above are an option then I want to know your overall height and seat height and I can calculate
If none of the above are an option then simply go to the Canyon site and use their PPS
But I want to be involved if I can. Let me know what you mean by 508mm and 380mm
I like the pursuit of mastery
Twitter - @TriCoachIan