DoD is 50% of (O-4 in this case) base pay: 6448.20/ month x .5 = 3224.10/ month
....
6c LEO FERS is 34% Of basic pay (base pay + locality pay + 25% Law Enforcement availability pay)... in this case GS-13 step 9 (w/ a +14.49% for upstate New York+ 25% LEAP) = 3792.98/ month, not including the FERS Social Security offset/supplement, which ends up being several hundred dollars a month.
Agree with your math, thanks for taking the time as I'm definitely not very familiar with GS retirement. I also agree with you that for those not "grandfathered" into the old retirement system (aren't guys that join in the next year or so still grandfathered?) the new system sucks.
My pushback:
A) Most SWOs that "stay in" (for DH) will retire at O-5+. An O-5 @ 20 gets ~ $4,250/month*. O-5 @ 24 gets ~$5,325/month. O-6 @ 30 is ~$8,158. I'm curious as to how this stacks up to GS-14/15 retirements (and also how common...do half of FBI agents retire as GS-14/15)? There's obviously no way to tell how far the OP would rise in the Navy or FBI, but I'm going with the law of averages...chances are he would retire as an O-5 or O-6 if he went career SWO; curious what that looks like for the FBI.
B) Military retirees get VA disability payments, which can be pretty decent for someone with a 20+ year, well-documented medical record.
C) With respect to the TSP matching you mention, while I assume you would argue that it's "free money," it's really not. A military member is free to spend that money on anything they want over the course of their career (a house, travel, beer, etc), whereas the GS has to, effectively, take a pay cut his whole career to garner the TSP payout.
*Note: the military retirements are approximate -- I'm too lazy to calculate out the true High-3 value, I ballparked it, they're pretty close.