I use a "pinch" in about 5 tablespoons of water. A pinch is equivalent to 23.5 micro-smidgins.
In other words, I am not very careful about the concentration.
But I do the next step very differently from most GA-3 users. I sow the seeds on top of the mix in a 3.5 inch square pot. Then I carefully water them in with a fine spray at low pressure. Then I drip the GA-3 solution onto the mix with an eyedropper. I usually apply about 36 drops of solution over the entire surface of the mix. This means that I don't have to go through the business with the filter paper and handling the tiny seeds.
Do not water from the top after you have applied the GA-3. If you do, it will all wash down into the mix where the seeds can't absorb it.
My method works well on Gentians, Physoplexis, Lewisia, etc. It doesn't give weak, elongated seedlings if they are properly grown at low temps under lights.
Leftover GA-3 solution can be stored in a sealed container in the fridge for a long time.