It's called the "shotglass method". You take a shotglass of pool water and a shotglass of distilled water (any two equal measures of water will work) and mix them then test and multiply your test reading by 2. If you need to test even higher you can take 2 parts distilled water to one part pool water and then multiply by three. Realize that the more you dilute your sample the more inaccurate your test result is but it will give you a ballpark figure. It is not a good idea to dilute more than 2:1 because your result will become more inaccurate the more you dilute. In your case you can get a good idea of Cl levels up to about 15 ppm.