My Xenia are directly in the path of a -tiny- powerhead. They do not rest horizontally due to flow, so I'd saw moderate current. They're under MHs and have acclimated wonderfully. I have so many now (started with 2) that they drown each other out for light. I'll be trimming them soon.
Anyways, they like a decent amount of light,
MH not required, and they like to have flow. Mine have always done excellent in that spot, and they're now spreading across the tank.
Sooo...
1. Light - they like it...don't be too afraid to give them too much. Mine thrive under a 250w 10,000K
MH
2. Flow - they like a decent amount of it. They like to gently sway in the currents, and they do indeed like moderate flow.
3. Height - They don't seem to care. I have them growing on top of a rock, with their tips 4" from the surface, all the way down the sides and front of that rock to where one touches the substrate with his stalk. If I had to pick, I'd go wherever they can get some decent light and a moderate flow, and that location is probably perfect.