FERS rule is simple:
a. If you retire at age of 62 or older with more than 20-year service, you will get retirement annuity of 1.1 percent [rate] of your high-3 average salary for each year of service. For example, your rate will be 33% if you work for 30 years.
b. The rate is different, if you work for Air Traffic Controllers, Firefighters, Law Enforcement Officers, Capitol Police, Supreme Court Police, Nuclear Materials Couriers, or Congress. Also, the retirement benefit rule for Military or Postal Office is different. Contact your agency HR in these cases.
c. You will additionally get annuity from social security and TSP depending on your vesting.