Yes, I agree with BTT and oldman47. We've had a lot of success with the ammonia calculator, particularly in UK where they seem to print a pretty accurate percentage of ammonia in the solution in the bottle (they don't in the US.) But as oldman47 implies, there's nothing safer than testing -your- ammonia and then starting low and testing the final strength you get in -your- aquarium. You'll note that RDD mentions this in the Add&Wait fishless cycling article that is our working document here on TFF.
So the way you test, that I think is good anyway, is to add 2 gallons of tap water to a test bucket. Put the correct amount of conditioner (per the conditioner instructions) in there first. Then use your guesstimate like RDD talks about, adding a counted number of drops that is intended to put you on the low side of 4-5ppm ammonia. Stir the water around thoroughly and test. Add more drops, noting how many, and test again. In this way you can arrive at a very confident amount of ammonia to add to the aquarium the first time such that it won't go way over. None of this has to be super accurate, just rough in its own way, but you want to avoid going way over, say to 8ppm, because that can be bad for your fishless cycle.
By the way, it is reported that a drugstore syringe with numbered graduations (miililitres usually) is the best type tool to have, two of them, one for putting tank water into your test tubes and one exclusively for squirting ammonia into your tank. (personally I used 2 glass droppers and just learned the number of "full squirts" but the numbered syringe would be even better.) First few days I counted drops (a lot of drops) but then I realized I knew what was "close enough", because after all, you are just getting your "green" in the right range.
Hope that makes sense,
~~waterdrop~~