Eye ball it. 30 years as a tradesman I have a good eye for level. Remember any change in ride height due to number of people or load will take you out of plumb and level. Change of mount angle from putting the RD up and removing it will change the level. Don't over think it, it isn't rocket science.
Close one eye, stick out the thumb and give it a go. Windshield radar detectors aren't going to be thrown off by the microscopic leveling differences. Just make sure the angle/cant is golden and youre set.
More power if you want it absolutly perfect, just not required.
Our hardcore testers are the ones who should be, and are anal enough, to get fair testing of detecors.