I woould spray it on last - on top of whatever other product you are wearing and want to heighten.
As far as how much - it can really vary individual to individual. I personally shoot for 20 mcgs but I am in my mid-40s so I dose higher for some things than younger guys.
I know that does not completely answer the question but I hope it helps.
It depends on the dosage of your other pheromones, IME. I usually keep it around 5-10 mcgs, but I would change it if I were only using it with p74, or if I were using it with IS.
For example, with 30-45 mcgs of p74, I’ll wear around 15-20 mcgs of p79, but with 2 sprays of IS2x, I might only want 5mcgs or 10mcg at most.