Thank you, mknauss. I have been scouring this site like I'm studying for state boards for days, lol. I have already learned a ton from you directly from reading others posts. Thanks for being so helpful to us all. To clarify, my pool itself it 4' deep, but the water measures 3.5' with tape measure. I would still use 90% of 18x9x3.5x7.5 = 3,800 instead of 90% of 18x9x4x7.5 =4,374?