Based on my crude math, 1/4 a gallon, for 30 days, will need approximately 8 gallons? At the price I'm buying at that is $40 per month, let's say $50 if you add an extra gallon for shock. Is that about that you pay for monthly upkeep?
~11,500 gallon in-ground 27 x 13 ft
3 ft in shallow, 5.5...