What is wrong with my OG calculation?

Looking at this recipe, they list an OG of 1.085:
http://www.homebrewtalk.com/f69/dogfish-head-90-minute-clone-260675/

For a batch size of 5.5 gallons with 70% efficiency (both stated in the recipe), I calculate the OG as:

2-Row Malt (16lbs @ 36 PPG, 25.2 Realized PPG):
(16/5.5) * 25.2 = 73.31

Amber Malt (1.25lbs @ 35 PPG, 24.5 Realized PPG):
(1.25/5.5) * 24.5 = 5.57

Together they give a projected OG of 1.0788, not the 1.085 given.

What is wrong with my calculation

 

I dont think anything is wrong with your equation. But, im not really a math guy

However, that difference can just be from the difference of the extract potential of that grain from year to year. From year to year, malt is slightly different so that change could account for the difference of gravity points. For instance if you just add one point of gravity to the realized PPG of both grains, you will get an OG of 1.081. Whos know what the extract potential was of that grain when the author of that recipe used it.

Best thing do to is add a little more malt to acheive your target OG, while keeping grain percentages the same

 

If you use UK pale 2-row with PPG of 38, the recipe (for me) comes in at 1.082 OG. Remember that these are small details - one point or two - and you can adjust the amount of base grain for your efficiency and system nuances if you like. The author even states that (but it's always true when copying someone else's recipe).