Do not know why OG calculations are so far off.

Guinness-Style Dry Stout
(5 gallons/19 L, all-grain)
OG = 1.041 (10.3°P)
FG = 1.010 (2.6°P)
IBU = 41 SRM = 44 ABV = 4.1%

Ingredients
6.25 lb. (2.83 kg) Crisp British pale ale malt or similar Maris Otter malt
1.75 lb. (794 g) Great Western flaked barley
14.0 oz. (397 g) Great Western roasted barley (500 °L) (crushed to powder)
8.35 AAU Kent Golding pellet hops (1.67 oz./47 g at 5% alpha acid) (60 min.)
White Labs WLP004 (Irish Ale), Wyeast 1084 (Irish Ale) or Fermentis Safale US-05 yeast

Step by Step
Crush the roasted barley very fine. Run it through a coffee mill or use a rolling pin to turn it almost to dust. That is critical to getting the right flavor and color with this recipe. Mill the remaining grains as normal and dough-in targeting a mash of around 1.5 quarts of water to 1 pound of grain (a liquor-to-grist ratio of about 3:1 by weight) and a temperature of 120 °F (49 °C). Hold the mash at 120 °F (49 °C) for 15 minutes then raise the temperature to 150 °F (66 °C) until enzymatic conversion is complete. Raise the temperature to mash out at 168 °F (76 °C). Sparge slowly with 170 °F (77 °C) water, collecting wort until the pre-boil kettle volume is around 6.5 gallons (24.4 L) and the gravity is 1.032 (8.1 °P).

The total wort boil time is 90 minutes. Add the bittering hops with 60 minutes remaining in the boil. Add Irish moss or other kettle finings with 15 minutes left in the boil. Chill the wort rapidly to 65 °F (18 °C), let the break material settle, rack to the fermenter, pitch the yeast and aerate.

Ferment at 65 °F (18 °C). Slowly raise the temperature during the final 1⁄3 of fermentation by 6 °F (3 °C) to reduce diacetyl levels in the beer. When finished, carbonate the beer to approximately 1 to 1.5 volumes and serve at 52 to 55 °F (11 to 13 °C).

Extract with Grains Option
Replace the British pale ale malt and flaked barley with 5.25 lb. (2.38 kg) Muntons pale ale liquid malt extract.

Partial Mash Option
The recipe becomes 1 lb. (0.45 kg) British pale ale malt, 1.75 lb. (794 g) flaked barley, 14 oz. (397 g) roasted barley, and 3.5 lb. (1.58 kg) Muntons pale ale liquid malt extract. Place the crushed grains and flaked barley in a steeping bag. Heat 5 quarts (~ 5 L) to 160 °F (71 °C), add grain bag, and let steep for approximately one hour. Rinse out the grains and proceed as normal, adding the extract and water to the steeping liquor.