In statistics finding percentiles relates to the standard deviation and something called a z-score. For normally distributed data the z-score represents how many standard deviations above or below the mean that group is a part of. The z-score for normally distributed data for the 90th percentile is 1.28. The standard deviation is then multiplied by the z-score to find, in this case, the shotlrtest height needed to be in the 90th percentile of this population. In this case to be in the 90th percentile your height must be 60.27 inches.