Fitbit Calories

Hello Everyone! I got a Fitbit Zip over the weekend and I am loving it so far! My only concern is that I've noticed that on MFP my calories seem to change everyday. I had it set to custom and I put in my TDEE -20% number and when I synced my Fitbit on Saturday I noticed that yesterday morning my they wanted me to eat a different amount of calories and same goes for today. Should I just change the number back to where I set it or should I go with it? It's hard to plan around meals when the calories are always different.

THANKS!