I posted this on What's Your FIRE Figure thread but I think it's most relevant here.
As I figure it there seem to be 3 ways.
1. 25x your yearly pension, so you pretend you have that amount and are drawing 4% or it. So if your pension is 20K per year you pretend you have 25 that amount or .5M in the "bank" as part of your stash.
2. The cost of an annuity for that amount. Not sure what the cost of an annuity for 20K would be but pretend you have that amount extra in your stash.
3. Consider it income. So if your pension is 20K and you need 50K to FIRE, then you really need to cover 30K, so your number will be at least 25x 30K, or 750K.
I think the last is the most useful, but I guess it depends on your circumstnces and what you are trying to figure out.