There\'s two aspects really: What is close enough that you say 'to all intents and purposes that\'s circular'? and What precision level do you not allow yourself to slip below? For the first, within 5 k is 'circularish', but for the second, I\'ll aim for sub 300m, but if I\'m feeling really bored I\'ll go for sub 100, but I\'m rarely doing anything that requires that level of precision! Personal best is about 80, but I was very bored....