On precision approaches 20-21" will almost always give the right ground speed in this airplane on an ILS except in strong headwinds. In the latter case use full throttle on the running engine. In either situation I would discourage against making throttle adjustments more than 1 inch either way. More than that and you have trouble keeping the ball coordinated properly. There is no way to fly well without already knowing the settings you are using to a fairly narrow range in such a touchy condition.