Reguarding your original question, methanol injection basically keeps your IAT's cool to keep you away from detonation. This can also be helpful in keeping you safe with not running race gas, or for a less than ideal boost situation like the one you are in with running boost on a stock bottom end. "How much" can all be determined by seeing your IAT's and how high they are with using the meth injection. It will not be the same for every car and every setup, it will vary.

Now, as far as running boost on a stock motor, I am sick and tired of saying my full thoughts. Just know that its not a good idea, and I don't think you'll last for a long time by doing even what most would consider a "safe" boost setup with a stock bottom end.

As far as meth/water, the debate goes on and on. Either will work, but personally I would reccomend a 50/50 water meth mix (thats what I am runnning with great results).

Also keep in mind that intercooling your setup will help aid cooling your IAT's, so doing this with the meth will allow for more timing, and greater boost levels which will equal more power.