Depends on what you're measuring. Is it PROM or AROM? What movement are you measuring? Just make sure you're stabilizing, and watching for those compensatory movements. Are those compensatory movements easily correctable with cuing? Or are they because of an actual joint/soft tissue restriction, or pain? If they compensate 2/2 pain, then you can document P1, R1, etc.
For example, if you're doing hip internal and external rotation in sitting, make sure you stabilize the distal end of the femur to prevent adduction/abduction, or further flexion of the hip. Then move the tibia into whichever direction for hip external/internal rotation until you have a firm end-feel, but without pelvic tiltng or trunk lateral flexion.