A slightly amusing relationship in the world of bike fitting: it happens that when my balance on my bike is correct, the bony protuberance below my knee cap is exactly above my pedal spindle on my came-one-the-bike 170mm cranks. Weird, huh? Certainly doesn't work for everyone, but doesn't seem unreasonable to use it as a place to start. That said, I did have to put on a 30mm setback post to get the clamp in the middle of my saddle rails. Maybe I have long femurs. Everyone's different, the reason that bike fitting is so much fun.