Round up to the next even multiple of 10 for CYA (example if CYA looks like 15, use chart line for 20, if looks like 21, use chart line for 30). If water is crystal clear, keep in target range, up to shock is safe to swim, and avoid below min. [FC/CYA][/FC/CYA
Quiz time! "CYA is somewhere between 20 and 30, closer to the 20 line than the 30" so according to the chart, what is your target FC range?
We generally suggest using a stabilizer/conditioner product of 95-100% CYA to raise CYA to 30, but if you're holding chlorine and not losing it all to the sun, and going to test FC and add often enough to keep in the target range, your method of adding via tablets is fine, just slower.
Thanks to
PoolMath, after putting in your pool gallons, and going to the bottom, we can see that each 8oz puck (throughout it's dissolving life) adds FC 3.9, CYA 2.4, and lowers PH 0.21 so if you have 4x or 5x pucks in there at a time, keep an eye on PH; maintain 7.2-7.8, and never go below 6.8.
Remember that you'll still need to add bleach during this process since it is adding CYA that does not go down, but FC that burns off, so pucks alone can't keep any pool sanitized for long.