So I don't know if others agree with this, but I always thought feet off the ground was a sign of a RES feeling comfortable / safe. Sedge used to be a very nervous basker, and I saw people post pics of their turt with feet in the air, but Sedge didn't ever do that. He was always nervous and ready to jump off.
Well he has gotten MUCH more comfortable basking in the past few months- and I think it helped that I added a second 'escape ramp' on the other side of his dock.
Look at those feet