I understand ratios between two values, but not ratios among three. I am not sure what it is saying. Let be the height of the fuel in the tank. Let be the diameter of the valve (approximately 9"). Let be the diameter of the tank. Now, you say . To me, that reads . But, why not just write ? What are the units for ? Do you know the diameter of the tank? Now, it might be the following:

If that is the case, then since is fixed, and I assume is fixed (the tank has a fixed diameter at all heights), . But, I have no clue if my assumption for is correct, as I still don't understand the ratio among three values (and just guessed that the first two should be in the numerator).