Tom, I am with you - that is why I stick with pacemaker/kalart combos. I can calibrate those exactly, and any top-mount rangefinder is not going to be as accurate without huge work in making a perfect cam for it. I go so far as to mate up one lens and one body, and shoot that way (handheld, with rangefinder focusing). I have 2 crowns and 2 speeds, all pacemaker/kalarts. one of the crowns is calibrated to a 135mm, the other will be to a 90mm. One speed is set to work with an aero ektar, the other with a 150. it makes for a lot of extra bodies lying around, but ensures that rangefinder coupling is spot-on. I have shot 2 lenses on the crown, one calibrated, the other not. The only way that works well (handheld) is if the non-calibrated one is fairly wide, thus forgiving depth of field. But focusing with the ground glass is clumsy and cumbersome vs. just using the rangefinder.