5-5.5 isnt bad, you know ! thats IIRC all the human eye can discern at the best seeing anyway.
I probably get down to about 3 or 4 - though thats a complete and unashamed guess, as I've never really thought about it !
however, last year we had a holiday in Pentewan near Mevagissey,Cornwall - the seeing was absolutely incredible - every clear night, the milky way was absolutely gorgeous !
the camp is right by the beach, so come twilight I took my eldest up there with some binoculars - needless to say she was impressed !
the next clear (hah!) night after we got home, I got so depressed - the orange glow of Wolverhampton to the Northwest, Birmingham to the Southeast, Walsall to the East, and Dudley to the West really p!ssed me off.