Hi Tsuki,
Based on my own experience with uprights and recumbents, if ind I get a better deal when riding a recumbent than when on am upright. I put this down to same eye level contact.
On an upright my eyes are above most car drivers, and even though in theory I should be able to see better, it does not work, and car drivers seem not tosee me.
Get on recumbent, at driver eye level, and its a different world--I can see "them " and get a better idea as to whether "they " have seen me! Plus I can see through any car that does not have tinted windows.
So that simple means that too high is not so good, and too low is hopeless in traffic!
Unless you really want to lean back towards horizontal, it probably means a seat height around the 15 to 18 inches is going to work OK.
But it is a very individual thing. Also, if you are talking of a two wheeler, seat height does not affect cornering anywhere near as much as it does with a trike.
However peddle height does affect cornering, and this is where a recumbent can score very well. With higher pedles it becomes almost impossible to hit the ground with the pedels until after you have fallen off--you can keep peddling right through a fast turn., and, with care, run closer to kerbs and not hit the peddles.

Steve G
Beijing