A healthy BMI is 18.5 - 25. Anything under 18.5 is underweight and I'd say to Iris that you're possibly anorexic if your goal is 17.2.
Everybody's different, BMI has its limitations. It's a formula that was given to us by a mathematician, I think, not a doctor. Expecting one formula to work for all of humanity is a little... absurd, isn't it?
Look at the other end of the spectrum - many athletes find themselves outside 18.5-25, which you've classed as a 'healthy' BMI. Would you tap any of them on the shoulder and say 'excuse me, but you fall outside a healthy BMI range so you're possibly suffering from an eating disorder also known as greed?'
I've met several people with BMIs ranging from 27 to 30 who are in enviably fantastic shape - low bodyfat, strong bones, lean physique. Yes, the BMI is a handy, if lazy, little tool for applying to a broad bulk of people, but surely there's always going to be people at either end who look and feel and operate better outside the magical 18.5-25.
There was a study from the World Cancer Research Fund that indicated that the risks of at least 6 types of cancer were heightened if someone had a BMI that was at the heavier end of the 'healthy' range, as opposed to the lower end. If it was closer to 25, the risk of bowel cancer was raised by 15%, and that of breast cancer by 7%. I'm not saying this should be taken as gospel, but it was detailed research and its conclusions were that occupying the lower end of the bracket carried with it a lower risk of cancer. And I'm not saying that being 'underweight' and outside the 'healthy' bracket is any better for your health than being overweight. I don't know what conclusions the study drew on that, but at 17.2 I'd still be closer to the lower end than someone with an allegedly 'healthy' BMI of 25.
What is it about a higher BMI (even inside 18.5-25) that apparently brings greater health risks with it? Fat? Lifestyle choices? Still much research to be done, I think, but it highlights some of the shortcomings of BMI as a measurement of health.