BMI is used to run statistics. And a BMI of around 26 I think puts you in the lowest mortality group.
Waist measurement is much more powerful predictor of health though.
And if you don't have time to read them here are the relevant graphs. It's average health care expenses by waist circumference. This was done with men and women in their fifties btw.
One useful thing with socialised health care is the great data mining opportunities ;)