If you are shooting with an M2, M4, or M5, you will probably find that the 35mm frameline will match the 35mm lens field of view rather well, with the possible exception of when you shoot at minimum distance. But with the M6, and beyond, to the present day, including the MP and M7, some of the framelines, including the 35mm and the 50mm--thanks to someone at Leica--got smaller (Honey, I shrunk the framelines). So when shooting with a 35mm lens, you get more in the picture than you saw in the framelines. Hence the popularity of the 40mm focal length, which is a pretty good match for the 35mm framelines in more recent Leicas. So some of us use 40mm lenses with our post-M5 Leicas, and the 35mm with our earlier ones.
Long story short, if you are using an M6, M7, or MP, just use your 40mm lens with the camera's 35mm framelines. If using an M2 or M4 or M5, just use a 35mm lens. Phooey on external finders for anything but lenses wider than 28mm!