Originally Posted by
urbanknight
I always figured having the limit screw too wide or having the derailleur too high would be like leaving an open door for the chain to fall through.
If you did that you'd wear a hole in the derailleur. That makes it shift even worse, and then you have to buy a new one.
The limit screw is only there to keep you from flipping the chain off while you're shifting. The derailleur height only affects the accuracy of the shift while you're shifting. When it's too high the flexibility of the chain comes into play. We want to minimize that effect. Optimum is 1-2mm. There are malignant cases where you'd want to go higher, but there's no need to go into that here.