If you get a bit of paper, like regular a4 thickness and put a peice between the bell and the spur, then push them together and tighten the screws it should set it ok. I tend to fold the paper in 2 and do it but just experiment.
When its set right the gears should be moving freely when spinning and if you hold the car on the ground the stop wheel moving the bell should move ever so slighly when you try turn it (only slightly though).
You'll soon learn whats right and whats not after a few clutch bells Just takes bit of practice, you'll soon have it sorted