I also think the 2" rule is somewhat wide. I am using the 2-1/16" in my yard, but on the mainline I'm looking at the prototypical distance, whatever that is. I'll be checking on that at northshoreline.com message board. The line was engineered to steam road standards of the time, so it's probably on the order of 12-13 feet.
Jay The Canada Goose is living proof that birds have cross-bred with cattle and rats.