I have 'tested' film speed for zone system purposes in various ways over the past 15 years. Most recently I have been using a Tobias densitometer. I have found the same pattern of EI being 2/3-1stop below published speed. I don't stick to one film, but I now have a set of EIs which I apply to certain films, regardless of the camera/lens/meter involved. I can't answer why this discrepancy occurs, but I have been persuaded by my test results that lower EIs produce a higher number of useful negatives. Alex