Not sure, I'm not thinking in percentages. I'd think no more than 10%.

If I were to crop it, I'd at least make sure that the rounded corners on the right side are never visible. A 2.35:1 frame that falls inside those borders should work. And remember, there are two different camera apertures during the film. I'd make sure the cropping fits inside the smaller one.

One guideline I'd give you: Make sure that each line of text in the opening crawl is visible from end to end as soon as it scrolls up into frame.