Hi Paola, people can (and some do) either but when your ‘personal’ TDEE leads to a fastday allowance of considerably less than 500kcal, as yours is (or more than in some cases), most people just go for 500kcal instead. It doesn’t make MUCH difference to your weekly calorie total and makes it easier to stick to 5:2 without feeling hard done by. As long as you are not desperate to lose every gram you possibly can in the shortest possible timescale I’d recommend you just stick to 500kcal.