Try this calculator.
You will have to type in your earnings history from your benefits sheet. Then, when it asks what you will earn in 2010, and what you will earn in 2011 and later, put in 0 for both.
Another method is to figure it the same way the SS folks do by following the method here
. However this is not a calculator, requires a lot of reading and arithmetic operations, and it is harder to do. Mistakes are more likely.
REWahoo is right that there have been a lot of threads on this, which would be worth reading as well.