That last part I understand!
It's something I used to do regularly when I went racing much more often. I used to add up the percentages mentally as I walked up and down the bookies' lines. So of course 9/4 equals 30.76% but as you say there's a personal element to factor in beyond that. I was maybe misinterpreting your "expected" win.
I'm quite partial, though, to backing place only:
a) if I really don't fancy the horse to win, and
b ) I'm getting 1/4 or 1/5 the win price.
It's very rare, though, that I'll take that kind of bet for a top 5 finish as the bookies tend to shorten up the place-only odds quite a bit more than maybe they should.