Shimano revoshift 7 speed use
A friction shifter allows you to continuously move the derailleur. The point of this on a front shifter (the left) is to trim the front derailleur. Trimming the front derailleur is when you tweak the position of the front derailleur to account for the change in chain angle with shifting, e.g. eliminate the chain rubbing on the cage of the derailleur. 

There isn't a "best" setting -- you'll adjust it depending on which gear combination you're in.