Your test kit reports total ammonia. Only part of that total is the form that hurts fish, and how big that part is depends on the pH and the temperature of your water. Put your three readings in and this works out the toxic figure.
Two published figures. The University of Florida's extension service: "Anytime the UIA is higher than 0.05 mg/L, damage to fish tissues can occur." The Southern Regional Aquaculture Center, for long exposure: chronic damage to gills and kidneys "as low as 0.06 mg/L", with roughly 0.6 mg/L given as acutely lethal. Neither is a cliff edge. They are the levels at which published work finds harm, and the sensible target in a home aquarium is zero total ammonia anyway, which is what the Merck Veterinary Manual's reference table asks for.
The formula is the one published by the Florida Department of Environmental Protection for calculating un-ionized ammonia in fresh water. It works out the dissociation constant from the water temperature, then the fraction that is un-ionized at your pH.
We did not take that on trust. We ran the formula against the printed conversion table in the Southern Regional Aquaculture Center's ammonia fact sheet and the table from the University of Florida. At pH 8.6 and 30 degrees C the published table says 0.2422 and the formula gives 0.2422. At pH 7.0 and 6 degrees C the table says 0.0013 and the formula gives 0.0013. At pH 7.0 and 20 degrees C, Florida's table says 0.0039 and the formula gives 0.0039. It reproduces the published tables, which is the only reason it is here.
It does not tell you your fish are safe. It converts one reading. Salinity changes the result slightly and this calculation is for fresh water. Species differ in what they tolerate, and a figure below the threshold with the ammonia rising day on day is a filter that is not working yet, which is a bigger problem than today's number.
For what to do about a reading that is too high, and for the rest of the tests, see what the numbers on your test kit mean.