The test I use requires no measurements at all. I sit on the bike in a hallway, leaning slightly against that wall. I do htis barefoot. I then place my heel on the pedal spindle. There is a very exact seat height where I can do this with my leg either completely straight or a slight bent at the knee without rocking my hips. Now, I find I use more knee bend than most. If I wanted less knee bend, I would do the same test wearing say a pair of near heel-less cycling shoes. Or once you have a bike that feels perfect, do this test and find the shoes/slippers with the right heel to pass this test. Now you can duplicate that height exactly on any bike (except you might have to make small adjustments for different pedals and cleats). The seat shape and softness will always be accounted for.

Ben