Find 5 typical non exercise days on FitBit site and see what the TDEE was on those days average. That's what you burn on rest days.
Find the BMR that MFP is using - Apps - BMR Calc.

Divide the FB TDEE by the BMR = multiplier for non-exercise maintenance.

Change your diet/fitness profile to the activity level that is closest to the same multiplier.
1.25 - Sedentary
1.35 - Lightly Active
1.35 - Active
1.45 - Very Active

Set weight loss goal to maintain. Set fitness goals to whatever you want, if you even look at the exercise tab fitness goals - most don't.
Save out. The daily maintenance on the summary page should be close to that non-exercise day TDEE you found.

Now go to MFP - Home - Goals - Custom - Change.

Take that TDEE figure * 0.85 for 15% deficit and enter that as your NET eating goal.
Change macros to whatever you want, and other goals as needed. Again exercise goals have NO bearing on the eating math.
Save out.

Set your sync options to negative and positive.

Only log your exercise like lifting or calisthenics that the FitBit won't be good as measuring, or cycling or rowing. Don't log walking or running as FitBit is good at that, and that will sync over automatically as adjustments to increase calories.

Eat your daily goal given.

And as this group advocates, go read up on what BMR means.

You have your eating goal set to 1800.

Your body would like to burn 2250 if you sleep all day.

You don't even eat back exercise calories.

See something wrong with this picture? I hope so.

Go read all the forum posts of those that thought they knew better and that bigger deficit and faster weight loss was better. When you factor in months of being stalled with no loss, it ends up being no where near as fast, besides being a whole lot more aggravating.

You got all the math right, just forget about the other BMR calcs besides MFP, because it's MFP you are adjusting.
So MFP is thinking your non-exercise TDEE is 2190 now.
FitBit is finding your non-exercise TDEE is 2155 on low side. Close as you'll get, and just fine.

You'll receive some negative adjustments on some days, probably mainly positive on many other rest days though.

So that is your rest day eating goal, 1831. You will receive adjustments now for exercise, and as suggested, be eating more.

Log any exercise FitBit is bad at estimating, according to their own FAQ, lifting, rowing, cycling, others like that which are not step based. Log those manually with MFP estimate, or HRM, or machine calories reported. Change the calories burned to minus the same deficit too.

Eat whatever the goal is changed to, whether that be from FitBit adjustments it's good at estimating, or exercise you manually entered.

And yes, you can do 20% easily. So that changes the eating goal to 1724.

Another tweak you can make to the system, is if you have very set workouts which are estimated well by FitBit, say you walk the dog or walk in general for set times each week. Very regular.
You can get bigger deficit by taking a typical 7 days, rest days and the exercise days FitBit is good at estimating both, leave out days it won't be good at. Be fair though, if your week is 2 rest days, to cardio days, 2 lifting days, ect, then bring together enough days for fair avg. Not high end, not low end.
Take the average of that now. Might call this FitBit exercise TDEE avg.
Run through the same routine dividing by BMR of 1753 to see what the activity level is now on avg. Should have gone up decently. Change the activity level using those mulpliers I gave in post above.
Same 20% deficit to create your eating goal amount.
Your adjustments both ways will be slightly bigger, negative on rest days, positive on cardio days, but you just took deficit on the whole amount.
Now still log the other exercise with deficit manually.
This method puts more calories in to the TDEE before you take the deficit, meaning it's a bigger deficit.

Close. Here's what you are trying to do in basically. Make MFP understand what your FitBit is going to report as your TDEE on avg, and take a 20% deficit off that amount to actually eat. Anything more or less will receive adjustments.

My first suggestion, using non-exercise days only, means you get a 20% deficit on smaller TDEE because no exercise included.
Then FitBit syncs across workouts and you eat those back - with NO deficit included.
Any manual workouts entered you would of course take the 20% off.

The above suggestion, using exercise and non-exercise days, means you get a 20% deficit on bigger TDEE because exercise is included.
Then FitBit syncs across some small differences and you eat those back, or actually lose some calories on rest days.

So multiplier of 1.39.
So pick MFP activity level of Lightly Active which is 1.35 - MFP should set goal to 2367, pretty close to the FitBit avg of 2449, as close as you'll get anyway.

So now FitBit 2449 avg TDEE - 20% = 1959 to set your net eating goal to.

You will eat or skip any adjustments that come across, set positive and negative syncing option.

So MFP thinks your TDEE will be 2367.
On days of exercise FitBit will show higher than that, and you'll receive positive adjustments which you'll eat.
On days of rest FitBit will show lower than that, and you'll have to skip some calories for those negative adjustments.

But now you are getting deficit of on avg 490 calories daily. And though eating back partial exercise calories, more on some days, less on some days.

Seeing as how FitBit does underestimate sedentary day calories, you'll actually be getting a bigger deficit than that on rest days.

And if you bike, you'll have to manually log those calories, minus the 20%, and eat those back.

Well, throwing 2 options at ya is confusing. This whole concept of tweaking things is confusing too.
Many luck out with default settings, many still are getting probably too much deficit and risking muscle mass loss.

I will say on the negative day adjustments, you decide. You know your avg TDEE with exercise is upwards of 2400, so on avg you would still be safe.
Like if it's the day after a late lifting session, the 24 hrs following is main repair time, and may be hungrier than a negative adjustment would allow. But you know at least the extra food is doing some good for the body, and you actually still do have a deficit, even if not as great as other days.

You'll start seeing normal adjustment types and know on rest days how much you'll have to give up.

Sorry to interrupt your topic, but am reading this & confused. You said TDEE -20% gives you the NET calories to eat? I thought that would be the gross calories to eat.

She is reporting the terms MFP uses.

This is also a totally unique setup. This is a mix of MFP style eating back exercise calories, and TDEE Deficit method with some exercise included.

Basically, her avg TDEE is based on a weekly routine of things the FitBit is good at measuring.

She is trying to adjust MFP to minimize the adjustments, but she wants them.

Yes, an easier method would be to use the FitBit TDEE stat, 20% deficit, and unsync the FitBit from MFP. The manually log any exercise that was not part of that avg TDEE stat.