One of these conditions is that the phone must be one with the car during the test : its sensors must perfectly "feel" the car's movements, and no other parasite movement or vibration. So you must pay close attention to the phone mount you use, and check that the phone position reach the PerfExpert mounting requirements. In the dyno test video tutorial, the phone is secured by a very strong windshield mount, equipped with a large suction cup and a very short, solid arm.
There is a much cheaper way to have your phone correctly secured in your car, and that allow you to immediately perform tests : using the original box of your phone, the passenger seat, and a heavy book!
Remove the cover of your phone's box and all unnecessary packaging
Put the phone inside the box and shake it sideways to check if the phone can move
Prevent any phone movement by wedging it inside the box
Make the power button accessible if needed
Put the box on the passenger seat, and wedge it against its backrest
To avoid any movement, you can stall the box with heavy objects located on its sides and/or in front of it