OK so I mostly shoot 50mm and I haven't noticed much framing error. Same with my Hexar AF but I do not compose so tightly. I know that a RF is not the best tool for careful framing but I do care to understand rangefinder limitations.

So I'm interested in going wider (28mm) but with an external viewfinder because I do not have the framelines (I have an M2/M3).

How "accurate" are these things? Are they just as accurate (or inaccurate) as in-camera frame lines?