First, which MF camera are you using? Is it manual exposure or does it have TTL flash? I doubt you can use your DSLR as a "flash meter" in this case unless you have a very advanced MF camera with TTL flash.
Mine is mechanical with just flash sync so I have to set it myself. My flash has a dial I can turn to set the ISO and range and get the aperture, it syncs at any speed. Or you can use the guide number formula:
GN = distance × f-number