In fact, you can use Pool Math in reverse to accurately predict the number of gallons in your pool if you are not sure. Using the FC Drop Test.....measure the FC. Add one gallon of illiquid CL distributed evenly about the entire pool to aid in mixing, and measure again after 30 minutes. If you know the percentage (I suggest FRESH Wal Mart 10%, not aged or stored)...and you have the pre/post numbers, you can back into the pool volume. This should be done only when the sun is near sunset, and it is critical that you have CC=0 (after a SLAM for example)....the pool cannot have any algae. pH between 7.4-7.8 is helpful too.