I would first check to see if infinity focus is correct. Then check the calibration of the rangefinder. The images in the rangefinder should align when the lens is at infinity and when the camera is pointed to a distant object. Some people use the moon, but I could never determine if this was correct. I usually took it to work and focused on a building across town.
I think that the sweet spot for the Tessar design generally has been from f/8 to f/11, possibly f/16 for a medium format camera.