Captain Obvious here...

As I was leafing through the old sheep judging post, something occurred to me - if you employ the pipe or tube to determine full-curl status, ensure that the pipe in use is cut off squarely, lest your view be skewed oval. I know I can't cut off a piece of pvc or abs squarely with a hacksaw - I'd use a chop saw set at a 90 cut instead. I wouldn't rely on the plain end from a piece of new pipe being square, either.

Just an FYI