Use your Fitbit calories burned plus your BMR to calculate a smart dinner calorie budget.
This uses the Mifflin–St Jeor equation. It is an estimate, not medical advice.
Use your Fitbit “Calories burned” number (TDEE so far today), your BMR, hours until bed, and what you have eaten so far to get your RealBurn dinner number.
The “500” is the built-in daily deficit, aiming for roughly one pound of fat loss per week (3500 calories).