add enough to raise your CYA 20 ppm. this way you will either be below 30 ppm or no higher than 50 ppm (range to shoot for is 30-50 ppm). If you test within range at the next test you are fine. If you are testing low then you should have enough to register on the test to get an idea of how much more to add. If the test still doesn't register then you had no cya and you should now have 20 ppm so add enough to raise it another 20 ppm and retest. You should then be around 40 ppm but should be no higher than 50 ppm.