What is the rule-of-thumb regarding five gallons of flow per linear foot of sewer?

This rule-of-thumb works as follows:  If you have a monitor basin with typical residential and commercial development you can expect to see dry weather flows on the order of 5 gallons per day per linear foot of sewer.  If you notice flow rates that are dramatically different, you should look for an explanation.  Are the flow monitor data correct?  Is the basin size information correct?  Is the basin boundary properly delineated?  This rule-of-thumb serves as a useful gut check.