k, 20 ppm without an algae problem seems a bit, well... excessive. You are trying to take care of the top 6" of the pool (and if it's just the grout - only a fraction of that area). Adding the extra water will cost you, adding multiple gallons of bleach will cost you, safely dumping 1000 +/- gallons of 20 ppm water will cost you, adjusting the pH and other chems will cost you and having the reagents in your test kit 'bleach out' (so that you don't really know how the water ballance is doing) could cost you BIG time. (Not to mention the oxidation of any metals around the pool, nor the possibility of anyone entering the pool clothed may end up exiting nude ...). Also, if 3ppm does the job, why 'nuke' the pool???
IMHO - getting 1 gal of bleach and a spray bottle would be a preferable option - it's easy enough to figure out how to mix the bleach to get to your 20 ppm level, and apply it where necessary.