Look at what I did for my gingersnap brown ale
I talk about how to use the nutrition info/sugar content to figure out how much to use.
An extract of it is also a good idea.
One other thing to consider for the end is to, if you're bottling, prime with it. You'd have to figure out how much sugar can be extracted from a fixed amount of the cereal, and you may need to supplant it with more priming sugar. You could steep the cereal and priming sugar in the 2 cups of water for awhile til the cereal breaks down, then strain the mess through a combination of coffee filters and strainers until you just get a liquid (it will be cloudy and yucky looking but in a stout or brown ale it won't be that noticeable in the finished product.)
Then boil that up to steralize it and use that as you would your normal priming sugar.
I would probably do a combination of the three, do some in the mash, do a vodka extraction and add to secondary of after fermentation has ceased in long primary, and then do it as a primer as well. The flavor should really come through.