I found that if I used the TF100 and knew the answers, I could see that the strip was in that range (except when they were totally wrong, like CYA at 0 when it was 30). The problem was going in the other direction.
Yeah, kinda pink with just a hint of orange, so CYA is on this side of 100 but not 50, so, um 75, 80, 90??? Then free chlorine is above 5 but below 10, chlorine is above 5 but below 10, um, what do I really need to do? Is there any CC? If I needed to keep FC 6 and 9 how do I know what to add?
Then do the TF100 and see CYA is 80, FC is 6 and I therefore need to add 3/4 of a big jug of 6% bleach. So, yeah, the strip was perhaps right but indicating a range when what you need is an answer is just not enough.
So when do I think strips are useful? If I had a SWG and wanted FC to be near 3, didn't care about CC, and needed CYA to be between 50 and 100 and would test for real with a test kit often enough that I would notice changes in CH and TA and CYA, maybe weekly or maybe monthly, depending on the area and water and particulars.
For the pool sitter to see if there is chlorine in the pool, fine, use the strips. Just don't do anything but add bleach for a week and I'll see what is real when I get back.
The worst thing is when your pool is NOT in the "IDEAL" zone with CYA of 30-50 and it says that TC of 3 is ideal and FC of 1 is OK. I went for awhile as a new pool owner, seeing that my CYA was "OK" at 100+ and FC was "OK" at 1, wondering why there was mossy green algae on the swim-out and steps.