Preparing for first AG batch. How to calculate efficiency?

So I am getting ready to do my first all grain batch and I am having trouble finding a formula for how people calculate their quoted mash efficiencies. Can anyone point me in the right direction on this? Is their a formula used to calculate efficiency?

I downloaded the trial of beer smith but didn't see tools on that related to calculating mash efficiencies.

Thanks,

Once you have entered

1. a recipe
2. and your Batch Size

you can click on the Brewhouse Efficiency button, enter your OG (this is the "after boil" gravity) to get your actual efficiency. This number you need to manually set in your Equipment settings to properly create your next recipe. Click on Equipment in the list on the left to make sure the setting is used for all future recipes. Note that the Batch Size needs to be as accurate as possible, and should be the amount that you finished with at the end of the boil.

Read these:
Understanding Efficiency - German Brewing Techniques
Troubleshooting Brewhouse Efficiency - German Brewing Techniques

Then use this:
http://braukaiser.com/documents/efficiency_calculator.xls