I like an image height that seems to reflect what I would expect the performance height to be.
For a couple examples:
On the title track of King Crimson's "Islands", there is a cornet that begins a couple of minutes into the track, that on my system, certainly sounds like it is at around 5' above ground. When I am sitting in my chair, the horn is definitely coming from above my ears.
Or, on Ralph Towner's great live album, "Solo Concert", his acoustic guitar seems to be right around the height of a person sitting on a stool. The guitar is coming from directly in front of me.