I'm the reverse. I have a small build and my 'normal' BMI is a fraction lower than the healthy range, right at the top of the 'underweight' range.

I'd go with clothing sizes or inches to be honest, if you're just aiming to get slim trim and terrific, perhaps that means you need to lose 8 inches off your bum and waist or something. Got any old clothes you want to fit back into?