I found you can use the pool calculator to add your chemicals, then wait a half hour to an hour and see if the results were as expected for your volume pool. Adjust the volume to make the numbers work out, then keep working that way. I had no idea what volume pool I had, so we measured and estimated 47,000 gallons. The pool inspector guessed 52,000 gallons. After 6 months of following the pool calculator and adjusting the volume to match my results, I ended up with my estimate of 44,000 actual gallons. I use that for all my chemicals now, and they seem to always be spot on.