Take a small sample of pool water (shot glass or some other unit) and add the same amount (or more) of distilled water and multiply by the following:

for 1 unit distilled water, multiply result x 2
for 2 units distilled water, multiply result x 3
for 3 units distilled water, multiply result x 4
for 4 units distilled water, multiply result x 5

To get a reading for 15ppm if your scale only goes to 3ppm you will need four units of distilled water added to one unit of pool water.

Peter