You are probably in the 60 ppm CYA range given what you described for testing. It's a hard test to get right and it has error of +/- 10 anyway. The minimum amount of Free Chlorine at that CYA level is 4.5 ppm (equivalent to the "Min" in Ben's chart) while a reasonable target (equivalent to a mid-point in Ben's chart) is 7 ppm. If you want to just make sure you never go below around 5 ppm, that would be OK, but this won't save you a lot.

It's really too bad it's not easier to change CYA levels since you could just try a lower level and a higher level to see if it made any difference (with corresponding lower and higher FC levels). You could take a large basin or bucket of pool water, dilute it with tap water to reduce CYA level and try that (adjusting FC level as needed). Then take your pool water in another basin (or the same one the next day) and add a little CYA to it and chlorine and see how the FC level holds up. It's not a perfect experiment because the depth of the basin or bucket isn't the same as your pool, but it's better than nothing.

Richard