KB004398: How is the Calculation of Peak Symmetry factor done? 

The peak symmetry factor is approximated by the calculation of the ratio of the Peak area before the peak top (Area, front) time and the area after the peak top time (Area, tail). The areas are corrected for riders (rider areas are subtracted, before the ratio is calculated).
Answer:
Peak symmetry = (Area, front) / (Area, tail)