I used a Nikon D3 with an f2.8 80-200mm Nikkor lens set at 200 mm. I first selected a neutral gray screen color that was as close as possible to an 18% gray card. So the aperture priority and spot metering of the camera was very close. It does not matter if your whole screen shows slightly light or dark. You are just looking for a consistent tint (or tinge) across the screen. If it is consistent you can calibrate it out using one of several screen calibration units on the market. I used a show shutter speed (1/15 of a second) on a tripod to eliminate any refresh rate problems. I used an Expo Disk to calibrate the color balance on the part of the screen that looked the most neutral to me. Further, because I was getting a moire effect in earlier posts, I used a polarizing filter to eliminate this problem. I have found that you get little edge fall off if you are a distance from the computer, which is why I was about 15 feet away with the 200mm lens and only filled the center 2/3 on the camera view with the computer screen. If you look back at my post you will see that my MBP had a more consistent screen than my i7. I do agree that unless you put all these iMacs in the same room with the same camera and settings that it is hard to compare one to the other, but that is not the intent. In the venue of this forum, with a little care, you can show a reasonable depiction of your screen. I am hoping that the sheer magnitude of this thread will get Apple's full attention and help those that have a real problem. Although my i7 has a slight tint (tinge) problem, I have decided that it is not enough to worry about based on my needs. That is my judgement.
Then I've underestimated your own particular efforts and capabilities.
Nevertheless, I don't see value in the end results.