From the University of Minnesota- Sports Medicine website they have a good picture of the measurement technique. Click on the link below and it should take you there or copy the link and insert it in your web browser to go to the site. Once you show this to a P.T., they should be able to take the measurement with ease.
http://www.sportsdoc.umn.edu/Clinical_F ... bility.htm
Hamstring flexibility is usually assessed by measuring the hamstring-popliteal angle. In this measurement, the hip is flexed to 90Â° and the knee is extended to a point where it becomes tight. The hamstring-popliteal angle is measured in relation to a line drawn parallel to the femur and the angle that forms between the tibia as measured from the foot. In this manner, a patient who can only have their knee extended to aright angle before the hamstrings become tight would be noted to have a hamstring-popliteal angle of 90Â° and would be considered to have tight hamstrings.