The “ideal” case is holding your arms out with your elbows bent 90°, the keyboard split in 2 parts such that your arms don’t angle in too much, and your wrists stay as close to a neutral position as possible. The ideal tilt is a bit up at the near end of the keyboard / down at the far end, and also “tenting” with the inner edge of each half elevated quite a bit above the outside edge.
To get the keyboard to that height involves, if you’re sitting and have typical body proportions, putting it on a tray just barely above the level of your lap.
If you put the keyboard on a desk any higher than that, then your forearms will likely be slightly upward tilted, at which point a negative tilt on the keyboard is going to cause you to bend your wrists downward, which is not ergonomic; so in that case, probably leave a bit of positive tilt. (But note that this is not the ideal angle for your elbows.)