In article <4asa20$sdu@cardinal1.Stanford.EDU>, poser@leland.Stanford.EDU says... (..)
>I plan on trying the EC-K screen to see if gives me enough "snap,"
>but I have been thinking that having a custom (ground glass?) screen >might be the only way to get what I want. My concern then is focus >plane. It doesn't do me any good to be able to see the image go in and >out of focus if it isn't in focus at the film plane at the same time. >What good is precision without accuracy?

Maybe the Canon bodies are similar in the way they locate the film
plane with screens that are changed through the lens mount opening
to at least some Nikons, in which the screen is held up against four
focus-plane locating points by springs in the frame under the screen
that holds the screen in (though that is no guarantee that the locating
points are in the correct plane [I have found a couple of bodies that were off a bit , but Nikon usually gets it right {I check!}]). If the "ground" surface is up, and there are locating points or surfaces, then the non-OEM screen should work if the dimensions of the rectangle are correct - though brightness may be affected by other considerations in the optical system. The lens rear element size makes a difference
in viewing brightness with some lenses, independent of aperture (which is why some Nikkors have unexpectedly large rear elements - they are designed to work well with the viewing optics). The prism materials and design greatly affect VF brightness, as does the viewing screen.
Hope this helps.