I like to use a syringe to measure out the volume of the pool water and distilled water. These are readily available at the drug store for dispensing childrens medicine.

I run a SWG which recommends CYA at 60-80ppm. Using Ben's Best Guess chart this means when I do need to shock, I have to increase Cl to 20ppm.

At this level, the amount of chlorine in the tap water is almost insignificant, so I use tap water instead of distilled.