I need to do a dance of joy at you right now.

This is what my dieticians say (to make a long story short: genetic condition, was VERY underweight for much of my life, still struggle to keep weight on above a certain point, thus will never ever blame people for losing fights with their metabolism as mine is equivalent to that of a squirrel on amphetamines) - the best comment I ever heard was, "It's very very ballpark. ...It's more like football-field. We can use it as a rough guideline once we know what someone's body fat percentage is, how their cholesterol is, whether they live at the gym, and so on. But on its own it's a number sitting on paper."