The R3A 40mm framelines will cover about 75% of the 35mm field of view. So, you can use it without the external VF but must be aware that your picture will include more than the framelines are enclosing. It's not a lot of space since it's all around the edge & you should be able to adjust to it. After all, the normal 35mm framelines only show 87% of what will be captured on film, so your reduction is less than 15%. (To achieve this the lines themselves are shortened by less than 10%.)
If you need more precise framing, this can be accomplished by using the R2A instead of the R3A. This will display the 35 mm framelines, which at 87% of the 35mm field of view are almost a perfect match for the 40mm lens.
Zeiss 35/2 Biogon, Zeiss 50/1.5 C-Sonnar, Rollei 40/2.8 Sonnar, Voigtlander 28/3.5 Skopar, Voigtlander 75/2.5 Heliar
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