Sub Frame Connectors

If you install connectors when the car is apart, they say to have the car perfectly level. What I don't know is, what is considered level? I have put a level on my rocker panels, rear floor pan, speaker shelf, and floor hump at the rear, all with different results. What are you supposed to go by?
If you aren't taking the car apart, you are supposed to install them with the weight sitting on the suspension as you would drive the car.. Make sure nothing is jacked up. A drive on lift is the preferred way in that case.