(1) start with a reasonable estimate of daily expenditure. (2) based on (1) predict the food intake to achieve the desired weight change. (3) measure food intake. (4) measure weight change. (5) update (1) based on (3) and (4). goto (2).
I can come up with any old crap as advice if, afterwards, I can tell the recipient "you need to update the figures until my predictions stop being wrong". That just means the figures I gave were wrong.