I would only put in enough to get to 1/2 of your target number. If you want to get to 40, then put in a dose to target 20. Let it dissolve, test for it (after a few days) and then add more. CYA is one of those situations where it's much better to creep up on your target number than to put it in all at once and overshoot it, since draining water is the only real solution to lowering it.