Installing quarter panels: how level does the floor have to be?

[scherp69]

I'm in the process of installing new quarter panels, inner deep tubs, outer tubs and trunk dropoffs. As I understand it, the car should be sitting on the wheels to get the best fitment, or have the car on jackstands where the wheels would be. Does this sound right?

Also, I was told to make sure that the floor is level. How level does it have to be?? I put one jackstand on one side of the car and one on the other side of the car making sure they were both the same height off the ground. I then put a long level on the tubing to see how level it was. The bubble is between the two lines, but closer to the right line. I lifted the level up slightly (less than 1/3") and the bubble centred. Is this level enough or should I put something under the jackstand to get the bubble right in the middle?

Thanks in advance.