BeerSmith calculates the FG based upon the extraction value of the grains/extract/sugars in the recipe, the efficiency of the system, the mashing temperature which dictates the amount of maltose which is produced in mashing versus higher chain dextrins which are mostly unfermentable, and the attenuation of the yeast strain which is being used.
If you are looking to adjust the finishing gravity of a given recipe, you can lower the mash temperature to lower the FG or raise it to increase the FG. If you are making an extract recipe, substitute some simple sugars (table sugar or corn sugar) for some of the extract. These simple sugars will ferment out completely as compared to the extract.
Another thing to look at is the strain of yeast you are using. Some strains do better at fermenting all the sugars which are available. These strains usually are talked about as being 'drier' in their flavor descriptions. Most yeast companies list the apparent attenuation for their yeast strains.
