I used heel on pedal years ago and then messed with it until it felt just right. It's not independent of setback and cleat position, though, so you need to adjust all three until you find the sweet spot. Formulae aren't much use as it depends on how much power you derive from your calves relative to your thighs and other factors that can't be measured (e.g. susceptibility to knee pain). I ended up with 95% of inside leg (saddle top to BB spindle), maximum setback and a fairly forward cleat position. I can tell if the saddle is 5mm too high or low.