Once you get your test kit, and learn to use it, you can use chemical additions to double check your volume guesstimate. You'll initially use that volume number to calculate how much of something to add (chlorine, acid, whatever). We here at TFP all use Pool Math for those calculations, it's an app,
check it out here. Test your water. Add the appropriate amount of a pool chemical to adjust your water's balance. Let it mix in for a half hour or so (pump running), then test your water again. If the second test shows the expected result for the chemical addition, your volume number is good. If the test result is off by much, then it's likely* your volume number is not accurate. Adjust the volume number accordingly, and repeat this process the next time you need to add a chemical. Eventually, you'll fine tune your volume number such that adding X amount of a chemical results in the correct corresponding test result.
* I say "likely" because other things can affect accuracy. For example, if you're using this method for FC and chlorine, you'd want to do this process at night, ideally a day or two after anyone has been in the pool, and once you're certain your pool is free of algae. Sunlight and heat and humans and algae burn up chlorine, so you want to eliminate those variables as much as possible to improve the accuracy of the two back-to-back test results.
If all that made absolutely no sense, no worries. It will... if you stick with TFP...
