Have a look at John Palmer's How to Brew online book. www.howtobrew.com

Off memory there is a section on how to calculate this. What you need to find out is a few specs on the grain you use, I have some of these that I can give you but I haven't yet collated all the info on my grains. But basically you can work out the efficiency of your system by working out the maximum possible amount of sugar that can be extracted and the actual amount you did extract. Hmm, bit confusing to try and explain here, ok, I just copied some stuff off a site below. Hope it helps.

Extraction rate

Calculating your extraction rate or mash efficiency/yield is useful for determining the efficiency of your system, and to determine your expected OG. The following formula gives the potential SG if the efficiency of your setup were 100%:

Malt SG * 8.333 * m

SG(pot) = -----------------------

V

Where Malt SG = extraction points/pound/gallon as specified with the malt

m = mass of malt in kg

V = volume of wort in liters

After collecting the sweet wort into your brewpot, take an SG reading, and compare this to the SG(pot) to calculate the efficiency of your setup:

SG(measured)

Setup efficiency = ----------------

SG(pot)

In the future you can use this result to calculate your expected OG, by multiplying the potential SG and the system's efficiency:

SG(expected) = SG(pot) * Setup efficiency

Just remember that in order to calculate the OG, the volume V must be the after-boil volume, because of evaporation during the boil.

Bit confusing I know. But I think the Maltcraft website has the malt specs you need.

Hope anyone else can help you out too.

JD