On my bass I messed around with different positions for about a year before I finally got something that I really like. Horizontal tends to collect on the flute body the spit that flies past the embouchure hole. That's no good. Also it can tend to stress the tenons. I find a more vertical orientation to reduce both issues. Not completely vertical, but mostly vertical. The 30* rotated toward you from vertical that you mention sounds about the same I use. That's on a bass - alto may be a little different.
It's common to have the top octaves go sharp with a curved head (bass or alto). I've read this is due at least in part to the fact that the curved head usually has a different rate and length of taper because the curve itself is usually a constant radius mandrel bend (no taper through the curve). Reliable sources have told me some alto flute makers position the tone holes slightly differently for curved versus straight heads.
It's common to have the top octaves go sharp with a curved head (bass or alto). I've read this is due at least in part to the fact that the curved head usually has a different rate and length of taper because the curve itself is usually a constant radius mandrel bend (no taper through the curve). Reliable sources have told me some alto flute makers position the tone holes slightly differently for curved versus straight heads.