I've been using 3 coals per bowl. The bowl itself is the one that came with the hookah. Egyptian, about 5cm in diameter. Tobacco I've used is mostly Al Fakher, and also Nakhla Two Apples. I've been packing the bowl with the tobacco quite loose, up to, but not quite reaching foil. The holes poked in an even distribution across the entire surface of the bowl, with a toothpick, about 8mm from each other. The three coals placed on the outside of the bowl, evenly seperated.
Hope that helps.