You can also use the CarlD (patented)( ) Shot Glass Method:
Mix 1 shot of pool water with 1 shot of steam distilled water in (say) a Pyrex mixing cup. Pour into the OTO cell and read, doubling the value. If it STILL is over "5"ppm (actually 10ppm), then...
mix 1 shot of pool water with 2 shots of steam distilled water. Now triple the value of the reading which, when at "5" means it's 15ppm. You can do it with 3 shots of distilled as well and then "5" ==> 20ppm.

However, the more you dilute, the greater the chance of introducing inaccuracy.