As another example I tested one of my screen shots I took after I painted my ceiling flat black out from the screen a distance of about 5 feet.
I ran the image all the way up my 4:3 DIY gray screen for this shot to show the change in ceiling reflection but it also shows at the bottom the 18 inch black bar I then get. When sampling this photo I get RGB values below
Flat black wall outside the screen area 1, 1, 1
Black/ gray bar below the image 14, 22, 25
Darkest area in image (hair above ear) 17, 20, 25
Brightest area in image (white trim on shirt) 222, 255, 255
Contrast is really the ratio between what is the best white I can produce while at the same time the darkest black. Its then reasonable to believe the projector can space out its CR between these two extremes. In the case of this example I'm doing pretty good with a range about 200 RGB points difference.
Before someone posts in about this method of testing I do understand its not the most accurate way to do this and the cam could have made many corrections to the image I took the picture from. But I do feel this image is close to what I (think) I see and also a method others could use when trying to compare image A to image B when thinking about a screen paint.
The below image is a 5 times blowup of the upper left corner of the photo and the inset block was cut and pasted from the self masking black bar at the bottom of the photo.
This picture contains the 3 black levels in the picture all in one blowup without the effect of the brighter face part of the image.