We're not yet FIRE'd (another 5-6 years I think), but here's how I do our planning.
Start with what we make today (these are not real numbers since I don't want to tell everyone my salary....but we'll use them for mathematical purposes).
$100,000 Gross
Then we adjust for things that will decrease. For example, say we save $22,000 per year and we'd stop saving at retirement. Then our house will be paid off the year we retire...so that's $18,000/year less. Then work expenses such as dressy clothes/shoes, briefcases, gifts at the office, expensive lunches out each day, etc....so we'll deduct $2,000/year for that. Auto costs will decrease...we have 4 cars today (hobby of mine), but we'll be going to 3 cars, plus reduced gas driving to work - we'll say $1k here. Also taxes will decrease since our income wil go down...so I did detailed analysis of tax reduction...say $4,000/year for this purpose.
$100k Minus
-$22k
-$18k
-$ 2k
-$ 1k
-$ 4k
______
$53k
Then we add on for things that will increase, such as health insurance (we will rehire before medicare kicks in), so add $6k/year. Utilities will be higher since we're home more, add $1k/year. Groceries increase due to eating lunch at home, add $1k/year. Travel costs will increase...we plan to take one additional vacation/year at $5k. Then we added in a few other things like increased hobby spending, higher home repairs (house is aging), and more trips to visit family nearby (driving trips...only 100 miles) , so let's add $2k for these items. Lastly we'll buy LTC as soon as we rehire at $5k/year.
$53k (start point)
+$ 6k
+$ 1k
+$ 5k
+$ 2k
+$ 5k
_____
$72k Total spending estimated
Each year we refine these as we learn more. Doing this we've found a silver lining. Although the above numbers are "made up", in real life we save 30% of our gross. So we can live on about 30% less with no other changes in lifestyle. So much for the "you need about 80% of your pre-retirement income" rule. Couple that with paying off a mortgage and so on, and we have some breathing room.