How to calculate potential Specific Gravity of Fermentables

Today I did another ten gallon batch of my Holiday Ginger Snap Ale. I use two boxes of crushed gingersnap cookies in the mash and I need to figure out the potential Specific Gravity of the cookies so I can calculate an estimated OG reading for my BeerSmith recipe. I did get an actual measured OG reading of 1.074 and could subtract that from the estimated OG without the cookies but would like to see for accuracy sake. Anyone know how to calculate this for any given fermentable?

 

If ppg (pounds per gallon) is unit for weight of soluble materials, you could theoretically sink ginger cookies in water and measure SG.
Similar to that, when 1 pound of sugar is dissolved in 1 gallon of water you'll get 46 ppg (it will dissolve and yield 100% of its weight).

 

Look at the label it might have info for grams of sugar per serving.
That's how I did it with a box of breakfast cereal.

 

The assumptions you make with this method are: 1. 100% of the sugars in the product are available for fermentation, and 2. 100% of the sugars in the product are able to be fermented.

This very well may be true for both breakfast cereals and for ginger snap cookies, but I thought I would mention this for the sake of discussion.

 

Similar to this. The potential of grains are determined by grinding it into a fine powder then mashing it to convert all starches to sugar. You then measure the SG of the resulting solution, correcting for gravity.

Here you could grind up the ginger snaps, mix into 1 gallon and measure SG, as no convertion is necessary.