Note that it has been suggested in some circles that the water can be directly added to the petrol by using a solvent such as acetone. However, I have not heard of anyone actually doing this!
From what I could figure out from my experimenting. Making a system that injects water into the intake worked but only if it was calibrated perfectly and it seem to be around 10:1 with the gas. My car was already running lean from other mods so it knocked easily without water when I had the timing cranked way up. I was never able to get it to stay set right and gave up on it since if it was to little it didn't do anything. More than 10:1 it killed the mileage and power. Mixing it into the gas might work out better. It would reduce the amount of gas going into the cylinder by the amount displaced by the water making it run leaner depending on the O2 sensor, and if the water mixes evenly then it would always keep the 10:1 ratio or whatever works best for the individual car.
Sounds like a good experiment to do one of these days.