For a bow front tank, measure the depth at the middle, the maximum depth, and at the ends, minimum depth. Then average the two numbers and use that in the calculator. This isn't exact, but it is close.The calculator doesn't work for bowfronts
For years we all wanted to know just what the size of our tanks was in gallons, primarily so we could figure out how much light we needed. Now we know, I hope, that it doesn't matter what the volume of the tank is. What counts are the actual height, length and depth - the length so we can match it as best we can when using fluorescent tubes, the depth so we can estimate how many fluorescent tubes to use to cover the substrate with reasonably even light, and the height, so we can decide what type of lights (T8, T5NO, T5HO, etc.) will give the intensity we want at the substrate.
Sorry to be off topic.