I have recently purchased a new medium format camera - Mamiya 645 1000s. From the results of my very first film I came into the conclusion that the add-on metering system of the camera does not work properly. In addition I own a SLR Nikon F100. Is there a way to use the results of the F100 metering into the Mamiya? Is there a way to find a formula for the metering in the Mamiya?
You can simply use your F100 as a meter — assuming you cover the same framing. Metering (and the f-number aperture system) is identical across camera formats. Be aware that different cameras — and film — may have a different idea of what constitutes correct exposure, so you'll want to experiment first and perhaps have a standard exposure correction that you apply.
However, you would probably be better off with a digital camera which could give you a histogram and real-time preview — or an accurate stand-alone light meter. Or, of course, getting the camera you do have repaired.