I'd just like to say that because something may be a little tricky to set up or manage, that doesn't mean we shouldn't do it. I think the research cited by Justin shows that turn radius affects walkers' times AND form (apparently nearly impossible to maintain form on too tight of a turn). Bob Letson, in an article from MN in 1986, argues convincingly that the same is true of runners. Here is the link to MN #18, as provided by Pete in the conversation cited by Justin: Lesson article
Bob's piece starts on p 17. It's really worth a read (and his accompanying drawings are quite a treat as well). Bob and I talked about this at length during a measurement conference in Seoul in 1984- he really put a lot of study into this question.
I would say, whenever feasible, go ahead and plan for a circle turnaround, with a tear-drop shaped set of cones. Explain it to your cone-setters and your course marshals. But have a backup spot, just in case.
I'm not sure the Guido Guys ever got a complete answer (and sorry if my impression is wrong). I would do it like this:
1. Mark a single point that would be turnaround if it were just a point turnaround (cone).
2. Decide on radius "r" of turn for big circle turnaround. (You have to make sure there's plenty of room to the outside for runners and sometimes, for vehicles).
3. Measure back a distance of (pi x r)/2 from the point you marked.
4. Use a cable or steel tape to draw a circle with the center at your new point. The circle will show where to put cones.
Example, if you want a 10' radius, mark your first point, then mark another point that is (3.14 x 10) / 2 = 15.7' SHORT of the first point. (15' 8" would be good). That point will be the center of your 10'-dia circle.