it's Height/Weight2.0-3.0 depending on the method and researcher.  Standard BMI is Height/Weight2 but the curve doesn't fit for short or tall people.  They are using a modified BMI table to come up with these numbers (some exponent between 2 and 3, like 2.6).  They pick an exponent and an ideal ratio, then solve for the weight at a given height that gives that ideal ratio.