Most of the software that is out there already calculate for this as much as they can, they take the average fermentability of a grain given to them by the manufacturer and it is accounted for in the software. However, as stated there are so many other things that go into attenuation that the grain portion is a very small factor, including temp, yeast cell count, and time as mentioned above.
OK, that's great information but leads on to....
Which number on the data sheet that comes with grain gives the average fermentability?
Thanks everyone for the answers so far.