The only correct way to check your drive line angle is with a protractor. The larger the working angle is the more critical that the working angles on both ends of the shaft be the same.
If not one end of the drive shaft is trying to turn faster than the other causing a vibration.
This will also be harmful to the drive train component's.
Only a experienced shop understands how to set these properly. If they don't have, or don't know what a protractor is, then I would not use them.