I just figure out how much AI and ad mixture is required for each gallon of mix. Then, I figure out how many gallons I'll need and multiply.
You're always dealing with equal numbers of mix, this way.
If you know (___oz) of AI is needed for 12gallons, you can divide by 12 to figure out how much is needed per gallon of mix. If your app. requires you to mix 10 gallons, you can simply multiply by 10 (AI oz. req'd).
Only place I can see this getting "sticky" is when you're figuring how much surfactant to add. You wouldn't want to double-up.
This is all assuming the ad-mixtures are compatible.