No, you have it exactly right - it is that simple.
I use 6 gallon pots, and I was doing it the way you describe, too. After all, if they're in the same size pots and in the same room, they tend to go dry together, or relatively so. I think next run, I'll pay more attention to individual plants, though.
So for example, I may have 2 plants that need GE/Tea and one that needs Cat/Tea, and maybe 2 Trans/Tea, and 2 just water. They're all in 6 gallons, so the drench measure is always 6 ml, and the Tea is always 0.6ml. I mix up a cup with water and 3ml Tea (for 5 plants). Then the same for the drenches - 10ml (a little light) GE, 10ml Trans, 5ml Cat, each in a cup with water. That's pretty simple. I put a gallon of water in a pail, dump the cup with the 10ml of drench into it, and feed two plants. Repeat with the Trans, then 1/2 gallon of water for the remaining one plant that needs Cat. Then mix and distribute the Tea (with another gallon of water) among the five plants. Lastly, water the 2 non-drench plants and top off the others.
You just calculate your measure for soil gallons and that's your dose.
[Edit] one more thing ... I don't pay much attention to whether the measure is 8ml or 10ml or 12ml. It doesn't really matter a lot. A lot of us end up using "plops" after a couple years.
So for other readers, ya don' hafta sweat exact measurements. The best part of this method is how hard it is to screw up.