Ahhhh - are you using their tub of ammonium chloride powder?
https://fritzaquatics.com/products/ammonium-chloride
We have been talking at cross purposes. Most of us use a bottle of ammonia dissolved in water, or Dr Tim's Ammonia which is ammonium chloride dissolved in water. These are all liquids not powder.
OK, so 1 teaspoon of powder in 100 gallons takes ammonia in the tank water to 4 ppm. You need to get it to 3 ppm not 4.
So 3/4 teaspoon per 100 gallons.
If your tank is 25 gallons you need a 1/4 of 3/4 teaspoon which is 0.18 of a teaspoon, or just less than 1/5 teaspoon.
To be honest, that amount would be much easier if you dissolved some powder in water, then used some of that water to dose the tank. It would be much easier than trying to measure small portion of a teaspoon of powder
Do you have anything to measure mls with? If you do, I suggest you make up a solution of 1 teaspoon powder in 100 ml water and keep it in a bottle with a tight lid. Add 20 ml of this water to the tank, that 20 ml will contain 1/5 teaspoon of powder.
Wait half an hour for it to mix in then test the ammonia level. If that gives 3 ppm, great. If it's less than 3 ppm add a bit more. If it's over 3 ppm, leave it but if you need to add it again, use less than 20 ml.