[Aren Siekmeier]

I'll speak to some our own team's specific experiences:

Our 2012 intake was driven by a 1:1 timing belt (16t XL sprockets) from a P60, and the center to center distance was calculated assuming circular sprockets and their published pitch diameter, as well the outer loop distance of the belt (not sure why). We ended up with a belt that could have been any tighter, which was probably not efficient or very healthy for the mechanism (though I have a relative lack of experience with belts). I don't think the tolerance was particular good in this case, however... And sorry I don't have more numbers, I have none of the CAD in front of me at the moment.

On an off season project last summer we had a chain reduction between 10t and 42t sprockets (#25 chain). We used the center to center distance given by a chain path in Solidworks (using the pitch diameters supplied by Andymark) that was a multiple of the pitch (.250). This was of course also assuming circular sprockets. We wound up with very loose chains that would not have functioned properly without tensioners (perhaps as expected because of the irregular 10t sprocket). The tolerance on this should have been excellent as we had the plates done by CNC (probably .001 or 2, but I wasn't directly involved in that spec).