Ian,

Thanks for the response. I did as you suggested with the wall and got a distance of 436 to the middle of the pad. I also remembered I had a bike fitting app on my phone, so I set the bike up and got this picture which seems to confirm that measurement at 436.2mm (the app does the measuring based on the wheel spindle to spindle measurement (which was 1020mm in this case).

This bike has always felt like I was scrunched up front to back, so if the pad x is too short that would help explain why.

I will also try to send you a few shots of me riding the bike in a PM, so they might also help you get a better sense of what might be going on with my position on the bike, and give you an better idea about what size would work the best.