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Cinestill 800T is a colour film manufactured for motion picture use. It originally had remjet on it, which served various purposes. It was also designed to be processed in motion picture (ECN) chemicals.
The "manufacturers" took the remjet off and then packaged it for sale to you, with the expectation you would have it developed in still film chemistry (C41).
The "ISO" of the film is stated to be 800 because with the remjet off the light can bounce around inside the film a bit, thus increasing how much exposure happens with a given amount of light.
As the film has been significantly re-purposed, and as you are going to get it developed in chemicals that are different than it is intended for, I would suggest that the 800 "ISO" number (for tungsten light) isn't exactly definitive.
If it were me, I would expose the rest of the film at an EI of 640. That is assuming you are shooting it in daylight. If you are shooting it under tungsten light, I would use an EI of 800.
Have it developed (in C41) normally. Then compare the results - you may prefer the EI 100 exposures.
One caution: most likely the results supplied to you will be in the form of scans or prints from scans. We prefer not to talk about scanning here on APUG, but the caution is that scanning itself can vary greatly in quality. It is very possible that problems with your results could come down to how well they are scanned.