If you don't know the concentration of the ammonia, you could take a container of a known size (1 gallon, 5 would probably be easier), add a pretty small amount (1/4 teaspoon), add it, mix it up a bit, and then take a measurement. if it's off the charts, you could do a simple dilution (say, 10 Tablespoons of clean water, 1 Tablespoon of ammonia) and then add a small amount (1/4 teaspoon) to another container full of clean water, and test again.
repeat until you get something in the readable range, and that should give you an idea (but only a very rough estimate) of how much you need to add to your tank. I'd probably try going somewhere around 1/2 of your estimate, measure, and then adjust.
Tablespoon and teaspoon aren't real precise measurement units, and I've heard there is quite a bit of variance, but a tablespoon is approximately 15ml, and teaspoon approximately 5ml.