I believe that the frame lines on the R2 are designed to show only 87% of the actual field of view. That extra 13% that the lens will see brings you to the field of view of a 40 mm lens. So, you can shoot with the 35 mm frame lines pretty safely. Just don't frame too tightly because you have no margin for error.
A couple of cautions . . . There is a reason why the frame lines are designed with some extra space around them. If you get your pictures developed with borders, some of the extra space will be taken up by the borders. Or if you shoot slide film, some of the extra space will be taken up by the cardboard frame around the slides. So, in these cases, you will have to leave even more room around your subject. Shoot a roll for practice & you will have a better feel for it.
If you use the 50 frame lines, think of them as defining the center of your picture, so frame tightly & be aware that you will have more in the picture than what you see within the frame. Some seem to prefer using the 50 frame lines when shooting up close. Try it both ways & see what you like. I prefer using 35 all the time.