When I "onstage" or "stage lip" I try to get the mics 1-2ft above the height of the front monitor wedges. Or conveniently about the height of those knee-facing guitar amps, and about the midpoint height of the drum kit. Not sure how much science there is to that, but it has worked for me. And well below a height that will be wacky for the performer.
If you're actually in the crowd, then yeah, you want to be at least head height, "head" meaning wherever most heads are (e.g., if everyone is seated, then at or just above where those heads are)