Interesting discussion... I've seen and used various charts of 'human factors' for layouts on some pieces, but can't cite them from memory. Based mostly on intuition, I cringed at your reference above allowing only 11" of 'knee room' at the end.

Then I put a couple of sticky notes under my desk at 11" and positioned myself accordingly for lunch. I assumed I would feel like JJ Watt seated at a tea table for 8-yr old kids. Surprisingly, it didn't feel that awkward. I think the limited knee room was offset by having arms on my chair. My sense is that if your forearms rest on the table comfortably, as is typical, or your elbows are supported by an arm chair, then you don't feel so much like you're perched on the edge of a cliff. -- YMMV

Not sure the above helps, but maybe you encourage your client to sit at a mock-up in the chair that they will actually use?

(I like the third, just because I like having someone sitting at the head of the table - traditional and honorable.)