You definitely need to test your film, making at least 3 exposures (-1, indicated and +1, preferably of a continuous coloured wall in direct unchanged light) - then you'll know where middle grey really is. One of those 3 exposures of the wall should be middle grey, then you can adjust your exposures accordingly next time you shoot a roll (i.e. a stop below or above indicated). I usually place Caucasian faces on ZVII (+2).