The issue I've found with my system and calculating water amounts is that technically the mash tun addition is variable depending on how much grain you have in there. This is because the inner filter section has both the gap underneath (which is fixed)- and at the sides (the height of which will be determined by the volume of grain in the filter). So I figure you actually need to know the volume of grain to work out the real amounts. Obviously it's just a bit of basic maths (enter π) to work out the volume of grain that will not be sitting in water. I could be wrong of course, and I also figure it's probably not too critical as long as you are over the volume of grain.