750-850 is what DC says it should be. I like around 700RPM in drive with the AC on, this way it won't stall backing up a load.
One member suggest 900 RPM. IMO that is a bit high.
800 sound about right. I noticed if it gets too low it will start hard (more cranking) and/or you may need to setp on the go pedal to get 'er fired up.
-Ryan