In my opinion, the easiest way to check chlorine levels which may be above the high end of your test method is to simply take non-chlorinated water (e.g. bottled water or well water (if it hasn't been chlorinated or ozonated)) and dilute your spa water until you get into a range you can read. If you can go to 5 ppm and if you think you are at >10 - <20 ppm, then if you add 3 parts of the "non-chlorinated" water to 1 part of your spa water (mix it) and then test the mix, you should be in the <5 ppm range. If you think you are under 10 ppm, you can simply use a 50:50 mix and see if you get a reading.
To be technically correct, you should also run the water you'll be diluting with "straight" to make sure it reads "0". If you don't get "0" or a very low reading, you should try another water to dilute with.
Sorry - modifying my post. I forgot to say that whatever your dilution ratio is, you need to multiply your results by that ratio to determine what your spa water's chlorine level is truly at. For instance, if you dilute 50:50, then you take your reading and multiply x2. If you dilute 1:3 (ratio of spa to non-chlorinated water) you would multiply x4.
Best,
Vermonter
