Are there any expert chemists on this list ? Can you explain how (i.e. the method) to answer the following question:
How many mL of 8.25% bleach must be added to a 470 us gallon hot tub to make the FC reading go up by 1ppm ?
If your answer is "I just use pool calculator", that's something I can do too of course, but I'm trying to find an expert chemist.
Once you know this, you can tell if your test kit is reading correctly or if your bleach strength is low. Knowing this allows you to calibrate some of the test issues you may see.