Hi egabe

For starters, let's check your pool volume. If your pool were 20 x 40 x 8' uniform depth, you'd only have 47,000 gallons. If it were 1/2 3' deep and 1/2 8' deep (straight drop to 8' at the midline), you'd have 33,000 gallons. I'm guessing you have a vinyl pool (those particular dimensions are typical of liner pools, more than concrete or fiberglass) and 20 x 40 vinyl pools with a deep end typically run in the 28,000 - 30,000 gallon range. That's a big enough error to matter.

Oops. Just checked -- you have done the chart, and have a FG + concrete pool. If it's a straight rectangle with a bowl shaped hopper, and the break at the midline, you are probably closer to 24,000 gallons. (Assumptions: 3' water depth through out shallow area; rounded bowl hopper; shelf under FG wall, along pool edges.)