I think most ITM musicians turn the foot away and don’t use any of the keys 
, but that wasn’t your question.
My hands are not very large, so the foot rotation position is primarily dependent on making sure that my small finger can reach both the C & C#; In that position, the Eb is still easy enough for me on my historic flutes.
With the R&R classic “claw-style” C/C# keys it is even possible (but not easy) to slide from (xx Eb to xx) C# to C (It’s easier of course to slide the other way). My problem there has to do with avoiding the accidental venting on my ring finger. And the claw on my flute is a little pointy, needing a small amount of filing.
Incidentally, for the first time I just noticed a little brass shim between the C & C# finger pads.