Just to futher elaborate and clarify, if I had a lens sled, with the lens out I would use 16x9 and it would scale as a typical non lens projector with a letterboxed presentation. Full appears to stretch the picture horizontally, which I only see a benefit with 4x3 material if you wish to stretch it to fill the screen. 21x9 stretchs the picture vertically to to fill the screen with the lens in place. So lens in light path and 21:9 for 2.35 content and lens removed from light path and 16:9 for 1.78/1.85 content and ignore Full.
Thank you so much for that Coolrda. That was very helpful will look at it again tonight.