attenuation is usually taken as a percentage measure of how much sugar the yeast can metabolize. Common american ale yeast strains can attenuate anywhere from 60-80 percent of sugars. Some yeasts are higher, some lower. All yeasts are different and their performance depends on many factors revolving around the health of the fermentation(temps, oxygen, digestibles, etc.). To determine the actual amount of sugar that will be in solution is a measurement called efficiency. This can be mash efficiency (sugars pre boil) and brew house efficiency (sugars post boil). Each of which is dependent on a number of factors such as mash pH, mash temp, water chem, and boil off.
There are a number of beer programs out there (you can find most of them around the forums) that given your grain bill, batch size, and boil off will tell you what you should expect for a sugar reading pre-frementation, use your yeast's estimated attenuation to see how much you will ferment out and that will give you an idea of ABV and residual sugars.