The formula below is used to compute percentiles of a normal distribution. where μ is the mean and σ is the standard deviation of the variable X, and Z is the value from the standard normal distribution for the desired percentile. Example:1  The mean BMI for men aged 60 is 29 with a standard deviation of 6.2  The mean BMI for women aged 60 the mean is 28 with a standard deviation of 7.here μ is the mean and σ is the standard deviation of the variable X, and Z is the value from the standard normal distribution for the desired percentile. Example: 1  The mean BMI for men aged 60 is 29 with a standard deviation of 6. 2  The mean BMI for women aged 60 the mean is 28 with a standard deviation of 7.