Trying to figure out just how precisely I have to measure when adding bleach according to Pool Math. For example, PoolMath says I need to add 172 oz and I have 121-oz bottles and 64 oz bottles. Can I just add one 121-oz bottle and guesstimate how much I need from a 64-oz bottle to reach 172 oz? or do I need to use a measuring cup to measure precisely? Is there some other short cut?