My fit requires a fully locked-out knee with heel on pedal. Some people like a few mm gap between heel and pedal. I think this has to do with preferred ankle angle at the bottom of the stroke. For fine tuning I rely on a feeling of power during pull-back at the bottom of the stroke. Too much leg extension and I don't feel powerful. Too little extension and my hams feel cramped and not sufficiently extended. I feel that difference in just a few mm variation in saddle height.