Hi,
Post the engine/trans combo so we can understand your situation better.
Generally, unless you have a fairly radical racing engine, you should be able to idle the engine at 750-800 rpm no problem...maybe 700 in gear with a C4...
The engine in my race car, for example, idles at around 950 rpm in neutral and makes nearly twice the HP of a stocker....
The number your husband read may have been for the stock emissions carb which had a idle cutoff solenoid which closed the throttle when you shut off the engine...
Please see this
thread for some more info on running on and the causes...
Good luck!
Pat
http://www.jps.net/binay/webdocs/strtmstng002_sml.JPG