There is a general rule (not sure where it started) that the i1pro has more difficulty with accurate readings below 1 ftL.
This can easily come into play when using the i1pro for CMS work, not just grayscale. Remember that blue is only 7.2% of the 100% white reading. This means that your 100% gray reading should be 13.8 ftL or more if you want to have more confidence with your blue readings with the i1pro. It is very easy for projectors to have difficulty reaching 13.8 ftL once fully calibrated.
So the low light accuracy of the i1pro can easily come into play with CMS, not just grayscale.
Now, having said that, this is what I did to resolve that issue. I got a nice size sample of my screen material and hung it much closer to my projector, roughly 3-6 feet away. (a 12" x 12" sample is plenty big). This raises the light output significantly reflecting off the screen material. Then, use your i1pro to read off the small screen material sample. Just make sure to keep the 100% white reading below 300 cd/m2 (87.5 ftL) to prevent oversaturating the meter. This allows for accurate readings, even with 10% grey readings and blue readings. (both of which can be problimatic with the i1pro).
Since the small screen material is the same as your large screen, the results will be spot on for the large screen as well.
I'll probably post this information in a seperate post to help others that have an i1pro, since it has worked great for me and I'd like to share the information.