I'm having a hard time making sense of which calories I should be following. For example, MFP says I have 506 calories left to eat for the day; FitBit says I have 630 calories left for the day. Also FitBit is saying I'm over on calories in vs. calories burned, i.e., I'm not in the "zone."

Are both set for the same rate of loss? And the same current weight? Usually my problem is the MFP allowance is lower than fitbit when set for a one pound a week loss, but that is because my net goal is at the mfp minimum of 1200. If your mfp net goal is 1200 then your allowances would be different because MFP wouldn't allow the full deficit you asked for. This, I am not sure. It might be because of your calorie burn at the last sync between MFP and fitbit. MFP calculates the fitbit adjustment by comparing your fitbit calorie burn to what MFP expects you should have burned at that point in the day. MFP also does some predicting what your fitbit calorie burn will be based on what you have done so far, your stated activity level, and how much time is left in the day.

But with the fitbit numbers, there are two things going on... There is the calories in vs. out. This is simply what you have burned so far vs. what you logged. Your calorie burn is not complete until the day is complete and all 24 hours are synched to fitbit. It is common if you look after you log dinner to appear to be over at that time, but you might have 5 or 6 hours left in the day (depending when you eat or log it). For following this, I think it can be helpful if you have in mind roughly what an hour of your BMR would be. I think fitbit uses a similar equation to the Mifflin formula MFP uses (for me it is two calories different). So if you run your stats through the Mifflin BMR calculator in the APPS section of this site, divide your total by 24 to get your hourly BMR. Most likely this is the least calorie burn you would see on fitbit for the hour (if you are moving around it will be higher). So if you know there are 5 hours between when you log dinner and 12am--multiply your hourly by five and add that to your current calories burned. Would that put you in or close to your goal? If so, you are probably on the right track. If you are quite active in the evening this will be a very low estimate. But if sedentary or sleeping much of that remaining time it might get you really close to where your total will end up.

The other thing going on is your fitbit allowance. This will end up calories burned vs calories eaten after all 24 hours are in place. But it starts you with an estimate and adjusts from there throughout the day. How it estimates depends whether you chose sedentary or personalized for your plan. If personalized, every day after 12am fitbit calculates your ":average calorie burn" over the past several days. Then it subtracts your requested deficit from that number. If your activity level is consistent this should put your allowance pretty close to where it ends up. But if you are sedentary after a string of active days your "personalized" allowance will be a little high that day. I like this method because it is kind of working with a TDEE based specifically on your recent activity history. It will evolve with your changing activity level, so someone who works their way up from 4,000 steps to 10,000 steps will see an increase in allowance to fit their improved activity (maybe, depending on how quickly they lose weight, they might just see a slower decrease in allowance than they would have if their activity stayed where they started).