I have a 18 x 32 pool.. shallow part (nearly 1/ of the pool) slopes from 3ft to 6 ft and then drops to 8 - 8.5 ft. Rectangular pool. With pool calculator it shows as 30,000+ gallons but I think it is not that much. As I need to buy 40lbs bags of salt. How many bags of salt do I need or exactly how many gallons do I have?