If you are metering flash, shutter speed only matters for the ambient (non-flash) light as long as the shutter speed isn't over the maximum sync speed.
If the meter is set at ISO 50 and we are talking about strobes, the shutter speed is NOT the thing to change. ISO 50 is one stop different from ISO 100. So if the meter says ISO 50 and f5.6, then you close down a stop to f8 for ISO 100.
You should be able to set your meter to ISO 100.
Set your camera and lights up. Take a meter reading. Lets say it is ISO 100 at f8. Take a note and take a picture. Then take pictures at different f-stops without changing anything else. f5.6, f6.3, f7.7, f8, f9, f10, f11. Compare and see which is properly exposed. If it is the one at f8, you are done and can use the meter as is. If not, figure out how many thirds of a stop you need to compensate to get an ideal exposure.