I think estimating an accurate BMR is one of most challenging tasks for calorie counting. If you don't eat enough, your body goes into starvation mode and will start loosing muscle mass.
I've been playing around with my caloric intake these past few weeks, it seems like I need about 2,000 calories to maintain my current weight. This is much lower than what I calculate as needing.
I'd like to get a more accurate BMR measurement, does anyone know how much these tests cost?