I am working on the same issue and posted about it in the FIRE thread. You've gotten some good answers already. My suggestions are to do some research at various financial advice sites and pay close attention to their assumptions to see how much they match your situation or not. For example, someone mentioned the 80% guidance, which is good, but does not match my situation at all, because I am currently saving a lot. more than it assumes and I
should therefore need much less than 80% of current income. I'm quite certain I spend nowhere near 80% or current income now.
My other suggestion, if you are not doing so already, is to get a good handle on what you're spending now (and on what). To me, that's the starting point, more so than current income. Once you know how much spend and on what, you can start fine tuning. For retirement, things that
might go away (or reduce) include
- expensive daily lunches (if you eat at home instead of 5-day take out)
- commuting costs
- certain kid-related expenses (depending on their age relative to your retirement)
OTOH, you might want to travel more and spend more on that (but maybe offset it a bit by going more off-season) or heath insurance might go up depending on your current benefit structure. And though you might no longer support children directly, there's a good chance if you have grandkids you'll want to treat them more than you expect.
Obviously there are plenty more things to consider in both directions. But IMO knowledge of what you spend now is the biggest tool in your arsenal to figure this out.