Why use the back edge of the saddle to measure cockpit length? Because of how I sit on my bikes...with my sitbones back on the widest portion of the saddle. That measurement tells me the MAXIMUM size of the cockpit...if I slide forward on the saddle, the cockpit gets a bit shorter.

The article at Competitive Cyclist on "French Fit" is one of the clearest explanations of why a "tall" frame is more comfortable on a four or five hour ride than a short frame. But, their measurements chart is not helpful to me. It is very subjective where an arm starts and ends. If I measure my leg length ten times, I get eight different measurements.

The only true way to get a "perfect" fit is from road testing a bike. If I were buying a new bike tomorrow, and I knew which specific model I wanted, I would just road test the two sizes that are in my range. I first test the tallest frame I could stand over without the top tube pressing into my crotch. Set it up with my preferred saddle height and bar height, and take it for a ride. Next, I'd ride one size smaller. Whichever bike provides the best "fore/aft" balance is the bike I'd buy.

I learned how useful "cockpit" length was when I was trying to figure out why several of my road bikes felt "perfect" during long rides, but I felt "bunched up" on one of my bikes, and "stretched out" on another bike. The bikes that felt "perfect" had a cockpit length of 31 1/2 inches. On a bike with a 30 inch cockpit length, I felt bunched up. On a bike with a 33 inch cockpit length, I felt stretched out, and I had too much weight on my hands.

So, now, each of my bikes is set up with an identical saddle height and cockpit length. Each of my frames is a different size, ranging from size 56 to size 61, but I have an identical fit on each bike.