IMO the light needs to be rigidly mounted and aimed level. I don't use low seat stay anymore, after seeing a friend seemingly disappear a few hundred feet in front of me. The road was gently curving right, just so his wheel obscured the light. Yikes.
Seat stay above the wheel, or below bag level on the seat tube or seat post. This can be difficult on a short frame. An alternative is one on each seat stay. Helmet is a great second location, but isn't consistently aimed correctly to be primary.