[jrc]

I put my M8 on a tripod with my Leica 21mm, so I could hold it steady, take a shot with known parallels, and examine the situation. (I put the edges of some windows right at the edges of the viewfinder.) With my eye held quite close to the viewfinder, I found that the whole field across the viewfinder is fairly close to 21. You couldn't hold it that close to your eye during normal handheld shooting, but even handheld, you can get a pretty good sense of what will be in the photo by looking at the full view. A little practice would help, and a dedicated viewfinder would certainly be more comfortable and probably more precise (although, depending on how far you are from your subject, a photo generally takes in 10-15% more than the bright lines show. In other words, the lines in the finder are not precise anyway -- you get more than you frame.)

JC