What was the reading on the color comparator for chlorine before you multiplied? Most only go up to 5, which limits you to reading up to 10 if you do 1/2 pool water and 1/2 distilled water.

Also, if you are currently at 18 for the chlorine, you are at shock level for your CYA (if that is indeed accurate - questioning the test strip accuracy, not you). No need to add any more bleach at this time (because of the potential for damaging your vinyl liner).