A nice find, indeed! 1.4x is a reasonable rule of thumb, figuring based on the nominal format diagonals. The film gates of my Olympus Pen cameras generally don't quite deliver a full 18x24mm, and there's some variation from camera to camera, so I don't think it's worth splitting hairs over.
Since you mentioned it, here's a series I made while out on a walk with my 50mm f/1.4 Super-Multi-Coated Takumar on my Pen FT.
Good luck, and enjoy!