Most water suppliers that use chloramine combine 4 parts chorine with 1 part ammonia (but this may depend on source water.
The concentration of chlorine and ammonia changes depending on how far you are from a disinfection point, and could be a minuscule amount. When I would test my tap water using a very sophisticated spectrophotometer, I might get an ammonia reading of 0.02ppm, and I was very close to the water plant/disinfection point, but the chlorine level might be 1.5ppm. Many over the counter tests might not be able to detect that ammonia amount, or considered as simply noise, (as your numbers show if your supplier uses chloramine).
I would think it more practical to use a DPD test strips made for chlorine to get the direct chorine concentration, instead of calculating, and treat accordingly. If you are 10 miles or more distance from the disinfection point, you may not need to add a dechlorinator.
But a simple pool test available at most big box hardware stores or pool supply stores will measure free chorine(FCL), chloramine (TCL, AKA total chlorine) down to 0.5ppm, and give you a better understanding of whether or not you need a dechorinator, and a few other important tests that would show valuable info.
If your source water is a river, chlorine disinfection rates may change from hour to hour depending on the water flowing into the plant, so if that is your case, I'd test chlorine levels before every water change.
If your water source is a large stable body of surface water (such as one of the Great lakes, or uninfluenced ground water, testing regularly may not be needed.
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