PIA computation (Primary Insurance Amount) links below:
http://www.socialsecurity.gov/OP_Home/handbook/handbook.05/handbook-0503.html
http://www.socialsecurity.gov/OP_Home/handbook/handbook.07/handbook-toc07.html
Determine your elapsed years: For retirement benefits, it is the 40 years between age 22 and 62. For disability benefits it is the number of years between age 22 and entitlement date to disability benefits. Could be as few as two years for someone who becomes disabled at age 24.
Take the annual earnings in those years. Convert the dollar amounts to today's dollars (indexed). Drop the low five if there are 40 years; a complex formula depends the number of dropped years when elapsed years are less than 40.
Take the indexed earnings after the dropped years and add them up. Divide by the elapsed years. Divide by 12. That gives you the Average Indexed Monthly Earnings (AIME).
Apply the percentages based on the bend points. The higher the AIME, the lower the percentage bend points get. Diminishing returns. Social Security benefits are 'weighted' to pay a higher replacement amount to the lower income worker.
Then you have the Monthly Benefit Amount.
Personally, I would run it through a computer program.