The BMI method of measuring someone for their body build has always been known to be flawed since it was first devised as it fails to take into acccont the muscle, fat, build of the individual.
A much better and more accurate approach is to measure the waist measurement of the individual and check that their waist measurement is close to 'HALF' of their height measurement. If you care to see the uploaded file it will explain more on the Ashwell Shape Chart (WtHt ratio)
It dosn't matter what measurement system you choose, imperial, metric or what ever, it's the ratio between Waist and Height that determines the risks.