The viewfinder on a rangefinder is really not the most accurate way to frame a picture. An SLR sees through the lens itself, so there is no difference. With a rangefinder you are looking through a window in the camera that is (usually) above and to the left of the lens itself. So your view is slightly different than the lens's. This is "parallax error." At infinity your view, and the view the lens sees, converge together so there is no parallax error. However, as you focus on things that are closer your view begins to diverge from what the lens sees. Usually you will notice that your pictures seem to show more on one side, and less on the other, than you had in your viewfinder (or framelines) when you took your picture.
All things being equal the way to eliminate that is to move your camera slightly up and to the left after you have framed your picture. In other word, try to put your lens in the same spot your eye was when you were framing your picture. However, to make things more complicated, things are not always equal. Most viewfinders on a rangefinder camera don't always show exactly what you are going to see, even without parallax error. So your attempt to shift the camera to allow for parallax error may not help all that much.
What I tend to do is to include more inside my frame than I want. I don't try to frame to tightly. That way I can always crop a little bit out later on if I need. But, you can learn how to get better framing by working with one rangefinder and one lens until you understand how the framing errors occur, and then begin to play with your adjustments.
A bi lengthy, but maybe it helps.