The HTH test uses OTO -- the color block only goes to 5.0, but OTO itself registers MUCH higher. What you want to do is keep your chlorine in the 'dark yellow with a orange tint' range (15 - 30 ppm), till there is NO green left. Once you get the chlorine up to that range, brush the pool completely. There may be piles or areas of algae that will survive high chlorine, but not being brushed + high chlorine. However, you do not want to brush those spots while the chlorine is low -- you could end up restarting the algae!