Advice on calculating daily caloric use

I'm trying to figure out as accurately as possible how much Kcal i can consume to maintain a stable defecit.

I'm using the Mifflin-St Jeor equation:

(10*weight [kg]) + (6.25*height [cm]) – (5*age [years]) + 5
Then you're supposed to multiply a scale factor according to your activity level.

I'm wondering which is actually more accurate, using these scale factors or letting the formula stand as it is and then adding the active calories from my watch which i wear 23,5 hours a day.

I'm trying to be as exact as i can and measure everything in and everything out with as much accuracy as possible.

Thanks.