The 85% coverage figure is at infinity, and it's incorporated into the R-D 1's finder frames as a safety device. Here's why:
The effective focal length of a lens increases as you focus closer -- in effect, the lens "zooms in" slightly at close distances.
Because of this effect, if the finder frames were designed to provide 100% coverage at infinity (which certainly would be possible) then when you focused down to 1 meter, the finder would show more than you'd get in the final image (because the lens had "zoomed in" compared to the finder view.) This would be dangerous --- you might try to compose the picture very carefully so some object would be on the exact edge, and then you'd be upset to discover that in the image it was cropped out entirely!
Since this effective-focal-length increase is an inherent property of lenses, every rangefinder camera has this type of safety factor designed into its finder frames -- yes, including Leica Ms, although I don't know the exact percentage Leica uses.
The only exceptions are a handful of cameras designed with viewfinders incorporating a "field size correction" feature -- the only ones I can think of are a few Konica models (IIIA, IIIM, Auto S2, Pearl IV) and the Polaroid 180. Of course, none of these have interchangeable lenses; to incorporate both switchable finder frames and field size correction would make the rangefinder mechanism ridiculously complex.
So, what's it like to shoot with a finder that's not 100% accurate? Well, of course it means that you can only make poorly-composed, crappy pictures, like those of Henri Cartier-Bresson, Elliott Erwitt, etc., etc. ;-)