There are several formulas and two major categories. First, did you retire with at least 20 years active duty or as a drilling reservist with more than 20 "good" years? Or is it a medical retirement?
If with 20 or more years active duty (and having started in 1988), you have to determine if you selected REDUX or not? So do you get a percentage on "high 3" or "redux"?
If you are a retiring Selected Reservist, its all based on your total number of drill points (active duty time is converted to 1 point per active duty day - ie 1 year equals 365 points). Plus, a Selected Reservist does not receive any money until age 60.
As you can see, there are so many variables that it is going to take a DFAS calculation to answered your question.
I recommend that you start with a visit the DFAS site and your service's personnel site. http://www.dfas.mil/
is the DFAS.
Hope that is helpful.