Calculate percent above 95th%ile for peds to classify obesity

Our pediatrician has asked if you can utilize the power of the EMR to calculate this for her: when the child is greater than 95% on the growth chart for BMI, calculate the percent of 95th percentile (e.g. 120% of 95 percentile). This will help her calculate obesity class 1, 2, 3. She is currently having to do this math manually. She would then like this to be in the vital signs app.