I with you Art. I try to "feel" my way through a set. At Pub gigs I can play my sets as long/short as I like. At festivals and most concert shows, the clock is the master! I find that when I write a set list and try to develop the set before I'm on stage, I always change my mind and let the audience and the music tell me what to play next - but I find that the mental preparation of developing a set list helps; it's like studying, you know mental prep even if you change your mind later - it also helps if an when you're stuck by nerves or distractions etc.