If you came across a recipe that asks for, say using 0.38 g of ammonia, do you know many teaspoons do you need to use?

Well, you do not have to use a scale for this.  Just a tiny pinch of ammonia is sufficient for use in most recipes.
When using measuring spoons, do you need fill the spoon just the rim or until it peaks?

Measuring spoons and cups are important for accurately measuring dry or liquid ingredients.  Unless specified otherwise in a recipe, the correct way to measure dry ingredients is to fill the measuring spoon or cup and then level the surface with a knife.  Don’t press or shake down the ingredients you are measuring.

How much is a dessertspoonful?
A dessertspoonful is approximately 15 ml.

Healthy Low Carb Diabetic Recipes

Treatment and Emotion of Diabetics.