I'm in maintenance tho' I've had some BMR/TDEE estimations based on previous scans/breath analysis.
The Scooby's Workshop calculator is an amalgam of several formulae and is very popular as it seems to deliver fairly good estimates of TDEE, particularly if people follow the suggested tailoring (tho' few do).
I projected from my last Dxa scan body composition results to modify the calculations for my present weight (assuming no loss of lean body mass: Dxa reports also use Miffin-St Jeor).
BMR is now <900kcals because of my low levels of muscle mass.
Projected TDEE is approx. 1000kcals on a sedentary day and 1275kcals on an active day (such as one when I kayak).
This is why I stick to 4:3 in maintenance: I find it easier to cope with 3 FD and have what seems like a reasonable amount of food on non-fast days. Assuming that it's true that we need to reduce our intake by 10% per decade as we grow older, by the time I'm in my 70s/80s it will feel more like I'm eating once a week.