Here is a method that was used by one of the forum boards members:

Here's the Jen-You-Whine CarlD Shot Glass Method!

1) Get a shot glass (like you use to mix drinks).
2) Get a gallon of steam distilled water--most mass-market drug chains stock it, and many supermarkets.
3) Mix one shot glass full of pool water with one shot glass full of the distilled in a clean container (like a Pyrex measuring cup).
4) Fill your test cell to the line with the mixture. Add your drops and take the reading.
5) Whatever you read, double it. If it says "3ppm", you have 6ppm. If it reads "5ppm", you have 10ppm.
6) If it's STILL seems like the chorine's too high to read, go to step seven....
7) Mix 1 shot of pool water with TWO shots of distilled water, and re-run the test using that.
8) Now TRIPLE your reading--if it reads "3", it's 9. If it reads "5", it's 15ppm.
9) Going beyond two shots of distilled to 1 shot of pool water is possible (3 shots, quadruple your reading) but you lose accuracy fast. Still if it's the best way of reading chlorine, then you have to do it.

CAVEAT: ONLY use this method to measure chlorine levels. Do not use it for the other tests you run, and do not use it with the FAS-DPD powder test--that goes to 50 to 100ppm of Free Chlorine anyway.