Originally Posted by I saw Elvis

Wow I didn't even know there was a formula. I've used the following method for 25 years: Put my racing kit on (including shoes) sit on the bike, clip into the pedals, move the cranks round until they are at '12:30' and adjust the saddle up or down until there is just a slight bend in my leg, at this point my knee should be directly over the ball of my foot, which in turn should be over the middle of the peddle.
It takes a few goes of getting on and off the bike, but it's always worked for me.
If the formula version works for you go with it, it's just the maths would give me a headache before I turned a peddle