If it is a watameter that you have they are very easy to calabrite, the wheel that you turn to get the distance read out, has a wheel on it that you turn to calibrate the rangefinder, just look at a distant object,with the first wheel, the one that has the distance on it, to infinity, then turn the upper wheel until the object is correctly superimposed, and the RF is correctly set, I have 2 or three of the watermeter's and they are often out of calibration, and the above has set them all, the setting's are very similer with many of the shoe mounted rangefinders
Richard