it dosent need to be an exact art you can aproximate it if you go over it will take a little longer to cycle your tank, but you will also have a larger bacteria colony. its all down to time, once you`ve worked out your initial dose of amonia, (say 5ml) when you see the amonia drop down to about 1 or 2 ppm add another 4 ml of amonia when that drops down to 1 ppm. But as i said its all down to time, when your having to dose the tank every day, then your tank is ready for fish...
also practical fishkeeping has a great article on fshless cycleing this month