I've read of a study of women workers pushing carts. Handles at 2, 3,
and 4 feet were about the same.
I mow half an acre without self-propulsion. My handle is set as high as
possible, but it's 8" below my elbows. If it were higher, it would be
easier on my shoulders.
My arms go down more steeply than the handle. It might be ideal if they
lined up. If the handle went down more steeply than my arms, pushing
could transfer weight from the big back wheels to the little front
wheels. Too much of that would probably be bad.