That formula is a rule of thumb type deal, though.  We are trying to decide what we think the real situation is in relation to the rule of thumb.  82% field efficiency is a good place to start for rough estimates, but obviously it is not absolute because every field and operation and even operator presents a different set of factors.    I don't think there are too many of our operations which do not stay higher then 82.5%. 

Our one planter records operating time, acres, and acres per hour for every job.  Corn planting ranged from 18 acres per hour to just a hair under 30 on the fields I planted.  It doesn't take much to kill the work rate on a small field.  Big fields are a little more forgiving because one stop doesn't automatically kill you.