Batch size used in BeerSmith color formula?

I was faced with the problem of counting the SRM color in BeerSmith. Can you help me find the error?
So, I create new clear receipt and add 2 grain into all grain receipt:

8 lb with 12.7 SRM
4.5 lb with 22.8 SRM
Batch size = 5.55 gal

So. Using formula from BS website (http://beersmith.com/blog/2008/04/29/beer-color-understanding-srm-lovibond-and-ebc/):

MCU = (8 * 12.7 / 5.55) + (4.5 * 22.8 / 5.55) = 36.792792..
SRM = 1.4922 * (MCU ^ 0.6859) = 17,69

But, in BeerSmith 2.1.02 I receive

Color = 16.93 SRM


What size of "Batch size" I must use? I tried to use "Batch size", Boil size (6.83 gal), Fine bottling (5.28 gal), Post boil (6.04 gal) - nothing help me.

Hi,
  I believe it uses batch size plus trub losses since some color does get flushed down the drain with the trub loss.

Brad