Nikon M2 extension tube

Hi all--
I can't seem to locate any info on the M2 extension tube...basically, I am trying to figure out how much exposure compensation I am going to need to do when using it. The meter on my F isn't working, I just need a ballpark number to start with.
Thanks,
Matt

extension over focal length equals factor , example, extension of 100mm with a 50mm lens is a factor of 2x so one stop