I thought I was going to get email notifications to replies in this thread but I didn't so I missed out on all of this discussion till now. Never thought it would go the direction it did, but seems to be some valuable info brought out. One thing I'd like to point out is the discussion of "verticals" in the frame as a reference for level. It is true that quite often vertical lines will be skewed in the frame, but one place where this never occurs is dead center. Each time I mount a camera on my rig, I zero the level by pointing it at something I know is a true vertical and putting the crosshair right on it. I hold it so that the vertical line of the crosshair lines up with the vertical line in the shot and then press the zero button. And it doesn't matter if the sled is tilted up or down. If it is a true vertical in the world and it is running right through the center of the frame, it will appear vertical in the frame IF the camera is level to the world. For me, this is the most fool proof way of calibrating the level of the whole system (camera + rig).
As to location of the sensor, it seems to me that on the main part of the rig is far preferable to on the monitor. My main reason for this is my concern about the need to adjust the monitor from shot to shot. Would that not necessitate re-calibrating it each time it is moved? Or is there circuitry in place on most of these electronic levels that would compensate for that?
Glad to hear Greg is coming out with an HD level. Looking forward to checking it out as soon as it is available!