and the other variable is the clocking of the derailleur cage plates to the chainring. I usually set the inner cage plate parallel to the chainring, to give it a better push. my only exception is when the chainline is screwed up and I need to swing the cage in or out a little to accommodate the out of line shifts.
It'd be interesting to see how others do this - I arrived at this method just by trial and error. There very well may be a better way to set the derailleur.