Hjortsberg: STOP. Stop thinking about one particular iso and one particular meter reading. All that will accomplish is a convoluted way of duplicating the camera's built in auto exposure function. That usually works but only because of the wide exposure latitude of b&w film. Instead, you want to intelligently utilize that wide exposure latitude.
Now it does require a bit of brain power but not too much. We are talking simple single digit addition and subtraction, not rocket scientist stuff.
First you must make a meter reading of only the key parts of the photo that you want to have detail, things like faces. You must eliminate those bright stage lights or dark background shadow areas that will wildly bias an average meter reading that includes them. Then determine the exposure for those key parts. Take in account aperture, shutter speed, and iso. If you don't understand how those interrelate, put away your camera for a few minutes and figure it out. If your aperture is computed to already be wide open and your shutter is as slow as you think you can deal with, then iso will become your variable.
Then consider the exposure latitude of your film. Realize that if you have to mentally bump that iso a stop or stop in a half to get the perfect reading in the previous paragraph, then you can just shoot at your film's normally rated iso and take advantage of the exposure latitude since one or one and half stops underexposure is within the acceptable range. This frequently will give you a technically better neg to work with, even if a bit thin, than if you extended the development time with larger grain and overall higher contrast. Realize that a slightly thin neg, even in critical areas, will not cause your final result to be totally blank, cause your camera to catch on fire, and you male heirs to be sterile the way some will lead you to believe.
Lastly, make a meter reading of the highlight and dark areas in the frame. Quickly visualize how much of your image will be totally burned out or totally black. Make sure that is OK with you desired intent. Remember that it is usually good to have a direct light source be totally burned out white and a large part of an interior or night image to be totally black. HDR is not your friend here.
All of the above takes only a few seconds and will save you large amounts of time and effort searching for some film that is not readily available and/or unnecessary keeping track of certain rolls of film to alter development time.
This was the way almost everyone did it back 25-35 years ago before camera automation "simplified" our lives.
