It doesn't look like your question got answered clearly. This is something I was confused with to start with as well and I know I've not been the only one. If I'm off on my explanation, please someone jump in and correct me.
We feed the soil, so the drenchs are directly related to how much soil you are using.
The instructions are for growing 6 plants in 7 gals of soil. Multiple that out and you get 1.5 ounces for 42 gallons of soil. 1.5 ounces is 45 ml. So that's roughly 1 ml per gallon of soil. I normally go with 1-1.5 ml per gallon depending on how heavy I'm feeding them.
Another thing that I've seen mentioned, and my limited experience utilizing it seems to reinforce. When the plants are small in relation to the pot, you should water twice between drenchs. When the plants are bordering on root bound, you want to water only once between drenchs as the plant will use up the goodies in the soil faster.
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