Designing for human variability frequently necessitates an estimation of the spatial requirements of the intended user population. These measures are often obtained from “proportionality constants” which predict the lengths of relevant anthropometry using stature. This approach is attractive because it is readily adapted to new populations– knowledge of only a single input, stature, is necessary to obtain the estimates. The most commonly used ratios are those presented in Drillis and Contini’s report from 1966 (Drillis and Contini, 1966). Despite the prevalence of their use, these particular values are limited because the size and diversity of the population from which these ratios were derived is not in the literature, and the actual body dimensions that each ratio represents are not clear. Furthermore, proportionality constants are often misunderstood and the interpretations of the results given when using them are many times incorrect. This paper expands on the traditional approach by: 1) explicitly defining the body dimensions, 2) defining a new set of constants for the 5th, 50th, and 95th percentile measures, 3) replacing stature with Body Mass Index (BMI) as the single input for select dimensions, 4) providing logit functions for easy derivation of any desired percentile and 5) creating an easy to use design tool utilizing the new constants. This boundary constant approach is shown to better model the range of variability exhibited in the data.