There is a way to calculate it out using a dilution equation but the most simple way is just to add a bit then test the water. Add about a capful at a time and test. when I did my last fishless cycle it was a 30 gallon and I think one squirt out of an eye dropper got it close to the 4ppm.
The poster above offered good advice on the 2ppm I only have experience starting a new colony then dropping fish in as soon as the cycle was done.
You'll probably end up adding ammonia daily. I think the accepted method of determining if a tank is fully cycled is when the bacteria can convert 4ppm ammonia to nitrate in 24 hours. So the 2ppm should be converted in the same amount of time.
It's also important to understand that your current level of bacteria was enough to support your rhom with feeding, waste production, and other factors considered. If you stock the tank higher right away you won't have enough bacteria for a completely cycled tank. If you see yourself immediately stocking the tank higher then it was for a single rhom then go ahead and dose the ammonia up to 4ppm and your bacteria count will increase to account for the higher ammonia levels. If I understand it correctly, 4ppm ammonia corresponds to 24 hours of waste production for a 100% stocked tank of any given size as the 4ppm is relative to tank size. For instance 4ppm ammonia in a 20 gallon tank is a lower total volume of ammonia than 4ppm ammonia in a 180 gallon.
