A fletching jig with a helical (or straight) clamp should be adjusted so that the feather or vane makes 100% contact with the shaft. This is done with the adjustment screws or dials depending on which jig you have. You can get away with a little more if you press the clamp against the shaft while the glue dries.

The ammount of adjustment necesssary is 100% dependant on how "fat" the shaft is. The fatter the shaft the more helical the shaft will take while still maintaining full contact between the shaft and the fletch.

I hope this answers both questions here.