If you're aiming to use a rangefinder for wide open and razor thin shots across the whole range of focus, then you'll need a very well calibrated and matched range finder and lens combination.
If you are planning to only use it for shots over a small range of specific distances, then you can get away with a less well matched range finder and lens, assuming you take the time to calibrate properly. The rangefinder on my Pressman was in rough shape and in need of a serious reconstruction to get full functionality out of it, but I'm able to get it close enough to allow me to take sharp images if I'm stopped down a bit and focusing in the rough ballpark of 15-25 feet from the camera. Focusing out side of that will have too much drift involved.
Lacking a working/semi-working range finder, then I think the easiest and most reliable way to use a camera like that is with scale-focus, then using a sighting tool that lets you estimate general distances. This method is used in marksmanship by measuring rough heights or width of common objects.
Specify your expected 'working distance', say 20 feet, then rig a sight that lets you compare the angular size of something, such as an average human head.
You can rig this with cardboard or tin 'flip up sights', and adjust the sight or the lens-board stop such that the when the width of a human head fills the 'range sight', then you know you're about the correct distance from subject.
This sight can even be as simple as a bit of thick foam/felt tape [like you might use on chair feet] and some safety pins pushed in. Easy to adjust and dial in your focus. - Set the camera up on a good tripod, carefully focus a subject on the ground glass, then place your pins to get your sight's distance. Experiment as is, then build a more durable version if you're happy with how it works.
This of course gets less reliable the thinner the depth of field you aim for, but can allow you to get more of the back and foreground out of focus than the hyperfocus style shooting goes for.