Joined: Dec 2004
Posts: 12,546
Likes: 797
From: Middle of da Mitten
Bikes: Trek 7500, RANS V-Rex, Optima Baron, Velokraft NoCom, M-5 Carbon Highracer, Catrike Speed
I'd aim for about 65 degrees from the line described by the power side of the chain. That's not a hard and fast number, there's wiggle room in either direction. Within reason, a shallower angle will let you shift a greater range of chainrings, and a steeper angle... won't.
If in doubt, measure the angle on some uprights.