Calc Extract Efficiency when using lactose.

[cajun]

I am trying to calculate my extract efficiency for an all-grain brew in which I added lactose.

I know all the factors for grains, but I also added 1 pound of lactose (to make a sweet stout). Given that I made 5.5 gallons of brew, can anyone tell me how many points of gravity were contributed by the 1 pound of lactose.

OG was 1.056

I had 9.5 pounds of grain and 1 pound of lactose.

[BillyBock]

The SG for lactose is 43 (sourced from the book, Beer Captured. Ok, now the calculations:
Lactose: 1# @ 43 pts = 43 gravity units (GU)
Total Brew: 5.5 gal @ 56 = 308 GU
Grain Contribution = Total Brew - Lactose = 308-43 = 265 GU
Theorectical Max from Grain: 9.5# @ 38 = 361
Efficiency: Grain Contribution/Theorectical Max = 265/361 = .734 (73%)

If someone could double-check my work, plz. Hope this helps, were you expecting this figure?

v/r
Bill