You are limited. As I wrote earlier: let FC dip below 10 and then you can actually check pH and adjust it. Your test block reads to 10, so when it's 10 or less you can trust the results and add to get to 20 again. But then you'll have to wait hours and hours (maybe a whole day) for it to drop low enough to read again. So instead of testing and dosing every hour or two so that the FC cycles between, say, 18 -20 ten times a day., you get to test maybe twice a day and your FC goes through a much wider range, like 8 to 20 twice a day.
Or just add 5 FC every day until your FAS-DPD tester arrives and then you can do it right.