The present 1000ES has vertical stretch modes of 1.33 and 1.24 or 1.25, I don't remember which. However, they won't work with a UHD or 4K input signal. Not enough number crunching power. The 500 or upgraded 100ES might rectify this but I don't know. The 1.33 anamorphics are 2K lenses and should not be used. Of course they can be, and they don't look that bad, but why bother even if you already own one.
If you must use an anamorphic lens, you need a 1.24w one rated for 4K. Will set you back $15K. Why bother?
BTW The Sony panel is 1.89 not 1.90.
I do see your point that if you use a 1.33w lens you will hit close to the ideal of about 2.37 if you limit the image width to normal on the Sony which is 3840 x 2160 or 1.78 rounded.