Good question... I've been in your shoes, and deduced that it was completely impossible to measure it.
Way back when, I emailed Dr Lee about it, and he said that it only takes a light dabbing, slight moisturizing of the scalp with it, and the scalp did not need a complete covering of wetness to get the chemical in. He reminded me that the spironolactone 2 is alcohol based, and alcohol is one of the best penetrating agents.
Still, I went through about 3 bottles of the stuff every two months when I was on it. Then again, it's just me to always overdo things...
If I were you, I would shoot for about a bottle a month. If you are burning through it at that rate, you're probably doing it just right. Adjust accordingly with your daily application.