Thank you.If you could clearly see the dot at 60, strained to see it at 50, and it was gone at 40, then your CYA level is 60. Always use the last number where you could *easily* see the dot. That was 60.
Running your FC as though CYA is lower than actual is risky, in terms of water sanitation. Running it a bit high is no big deal. Always round up.
It is possible, with the rains we have had as well.with CYA at 40 and dosing for 60 CYA will lose chlorine faster than having the CYA at 60.. We have had these clouds and rain for weeks now... still continues, I think Tuesday we may see some sun for a few hours... Even if it is a little cloudy the test should be fine...
losing 3.5 FC your CYA might be a little lower than 40???