You probably have done this already, but what I did was took the percentage range given to me by the AAMC for each section, assigned a mid-point percentile between the high and low number as the number representing that section, and used that number (e.g. if your verbal was a 10 and the range was 85-75 percentile, use 80 as your percentile score) to calculate the averages of all three sections. I wouldn't include the writing sample score in this calculation since most adcoms won't either. In other words, if the mid point numbers for each of the three sections are 85, 80, and 75, your overall percentile rank would be 80. Hope that helps.