Thing is, I wouldn't want the water/meth flow to be proportional to fuel at all times. There's no need for it until boost starts building. Subtracting a few milliseconds from the car's injector pulse width seems like a pretty straightforward way to get meth to come on gradually when needed, and increase with fuel flow.
I don't see time being an issue with a table-driven system, unless the someone wants it to be. You could set the table to behave just like a simpler system and then tune everything else around that.