The way it was explained to me back in 2002 with DOT/DHS:

• Take your top 3 years in earnings and average them together.

• You get 1% of that average for every year you serve. 15 years? 15%.

• That jumps to 1.1% at 20 years. 19 years? 19%. 20 years? 22%.

• This caps out at 33% for 30 years.

• Additionally, you get Social Security.

• Additionally, there's the TSP. (USE THE TSP)

• The three separate incomes streams should handle your retirement.

This is one of the reasons you'll see Republicans want to "reform" the system by expanding it from top 3 years to top 5, because the more years you're averaging, the lower that average is going to be, thus saving the government money that they can spend on things more appropriate to the Republican paradigm.

You'll also see various schema to try and tap into the TSP in some way, shape, or form, for the same reasons.

So the answer to your question is "Take the three years you made the most money and average it. Divide it by 3. That's your yearly pension. Divide that by 12 and that's your monthly pension."

Someone who ended up with their three best years being 110k, 120k, and 130k? Their average is $120,000 a year. After 30 years, they're looking at $40,000 a year pension, or $3,333.33 a month, give or take.

That's it in a nutshell. What it'll be in 30 years? I won't even hazard a guess.