I adjust based on crush quality, not gap measurement.
Well, I'd say crush quality and still having the ability to drain the tun in a reasonable amount of time. Another thing that Denny reminds me of, is the speed at which the rollers are spun. If you spin fast, you might have to increase the gap a bit and vice versa. Another variable is if you condition the malt prior to milling. All of these factors suggest that there isn't a preferred gap. However, there is the gap that works for you.
Just to be more complete, I crush very fine and have never had a runoff problem. I also run the drill I drive the mill with as fast as it will go and I don't condition the malt. Yeah, counter to pretty much everything oter people do. But I get great efficiency and IMO great beer.