Exposure Series - Kodak EPR 64

This film is expired in the early 80's and not easy to handle. However, I still have a few dozen rolls of it in the freezer and I need the correct exposure time for this film. The EPR has a film speed of ISO 64. I measured with ISO 50 and than I exposed with the correct aperture, one and two stop over. The best results are with ISO 50 plus one aperture/stop. Next I try to develope one of this films as E6.