There's no hard and fast rule, you just want to adjust it so there's a tiny bit of play between the cb and spur. Some time ago we found that Losi's liked a little looser mesh, it helped prevent teeth being broken off the cb (the losi flexed and has unique cb's). You can use a zip lock bag between the gear teeth to help set the mesh but you'll get a feel for it, ask guys at the track what they thing of your setting too.