Hmmm... That's a good question. I have never thought about developing this film in E6, so I'm not sure what its "box speed" should be. There is no speed printed on the can and there is no speed referenced in the spec sheet from Kodak. It's 30 years expired as well, so that effects the speed. I shoot it at iso 50 though I have read about people shooting it as low as iso 8. I think that generally duplicating films were slow in case you wanted to dodge/burn. I like the "pushed" look I get at 25-50, so I haven't experimented much. Hungry Mike has a roll of this as well, so maybe he can chime in and let us know how he is using it. There is a wide variance of how people expose for x-pro, from +2 to -2 stops. If I were starting with a new stock that I wasn't familiar with, I would just start at box speed. Maybe I would bracket a couple of shots (but probably not). Satish (Indofunk) is sending me a couple rolls of waaaay expired Ektachrome (yo0u can see a couple of his shots with it above). He shot those about 1.5 over box speed and while I appreciate the aesthetic, it isn't quite what he was going for. So I am going to try a couple of experimental things and see if I can get a different look out of that film.
I'm not sure any of that is helpful, but I look forward to seeing your x-pro shots when you get them.
Thanks,
James