Welcome to HBT & homebrewing!
I've not tried that recipe, but it looks like the site caters to Australian brewers? Depending on where you are, it may not be so easy to find a can of Cooper's Stout LME, etc.
However, I think it is always a plus when you can try a recipe that somebody has posted about.
I did not see any hops on that recipe, so I assume that the Cooper's can is pre-hopped and probably about 5 lbs or so?
The ABV really comes down to how much fermentable sugar is put into the beer. The math on computing it is not bad at all, and I do recommend John Palmer's How To Brew website for some good info on that.
Adding raw sugar contributes fermentable sugar, and nearly 100% by weight. Extracts usually have a fair proportion of unfermentable sugars in there to give the beer some flavor & body. So, 1 lbs of extract will yield less fermentable sugar than 1 lbs of raw sugar. And that is a good thing.
For a stout, many folks prefer a firm body, so you don't usually see lots of sugar additions. You want the unfermentable sugars in there.
As for the specifics of the ABV, if you can find out the weight of the extracts you plan to use, I recommend using some of the many free recipe sites (e.g. http://www.hopville.com ) to input your recipe, and they will give you a good estimate.
Another good way to go when you are new to the hobby, buy beer kits form the many online homebrew stores. Once you get comfortable with a couple of kits, you can start exploring out from there.