I believe the correct EI one chooses depends entirely on how one's meter is calibrated, and on one's own subjectivity. In other words, it is futile to ask anyone else what their working EI is, or for anyone else to suggest a working EI based on their own meter - unless, of course, you are both working with the same type of camera or hand meter. In this case, one could only presume that two similar meters from the same manufacturer would be calibrated the same (and I find they usually are), but even here, there's always an exception.
I can only recommend that you run a couple of rolls of test film, rating the film @ 40, 50, 64, 80, & 100 on several different "normally" lit scenes. Then, on a light table, choose what you consider the best exposure from each set. Use a quality loupe to be sure you have detail in all the highlighted areas where you want detail. If you are using studio strobes, do the same thing with your strobes on an appropriate subject model. You will most likely find that all or most of the choices you make will be the same EI setting. That should be your normal EI setting for this film.
I personally have owned cameras and hand meters that used EI's of 50, 64, 80, and 100 for Fuji Velvia 50.
With my PME51 Hasselblad meter and focusing screen, I had to rate the old Velvia 50 @ 100 EI in order to obtain a "normal" exposure. With the new Velvia, which I just recently shot my first few rolls, I found EI 100 to be just slightly darker, but still very useable with good detail in the highlights. However, I will rate it @ EI 80 next time.
Also, I've always been amazed at how many photographers do rate this film @ EI 40. In all the meters I've ever owned - Nikon, Contax, Mamiya, Hasselblad, Minolta, Sekonic, Gossen, to name a few, not one has ever required me to use an EI slower than its rated 50 speed. But I'm very careful with my chosen EI's, that under normal lighting conditions, I will not encounter any blown-out highlights.
