Keep in mind that any driveway/pad will have a slope... they're supposed to, to shed the rainwater. So will most parking lots, etc, that you might think will provide level ground.
Try this... using your trusty transit, adjust pads under where the jacks will go, until they are absolutely level with each other. (Never mind what the actual ground is doing.)
Then use that "artificifical level" to do your calibration.
IanP, the man from Scotland
Jay Feather 19XUD, pushing a Jeep Grand Cherokee