Just wondering...is there a formula or rule of thumb for estimating the amount of CYA drop due to rain. (I realize that an actual test is the only way to be sure.) At last check my CYA level was around 65-70 (checking weekly since opening and adding each time to get to 70). We have had about 4inches of rain in the last 7 - 10 days, so I'm thinking the level has dropped due to dilution and having to pump off about 2", but since my test kit has a limited number of CYA tests, I'm trying to estimate CYA without having to recheck every time it rains. Any thoughts?