On the SS.GOV site you can find a table of the index factors for each year (starting in the 1960s I think) for someone born in 1950. Anyway, if you print off a copy of your SS earnings from the web site, you can multiply your earning for each year with the index factor and get your inflation adjusted earnings for that year. Once you put in all your data, sort the results to get the highest 35 years of adjusted earnings and get a total. Divide that by 420 (35 years in months). That's your avg indexed monthly earnings. The site will explain about the exact formula, but if you take about 1/2 that amount you be will close to your monthly benefit at age 66.
I have a Excel spreadsheet, but it is easy enough to create your own.
The SS site now has an index table for someone born in 1951.
If you want a copy of my Excel spreadsheet (it's a 2002 version) email at
JoelJohnson530@gmail.com and I'll send you copy. You will just have to enter you own numbers of course.