Possibly some clarification needed here in the use of external viewfinder: You still "focus properly with the original viewfinder", then shift your eye to the external viewfinder for final framing of the scene. The 28mm does not require guessing focus, though like any other it can be used that way if you wish.
With the Voigtlander 12, 15, and Snapshot Skopar 25mm, there is no RF coupling, so with these guessing focus is necessary. Same generally with SLR lenses adapted to the M mount.