Deano,
For the PA28-140, 180 and Arrow II;
Full Tank - 25USG
Tab - 17USG
Base of the Outer Tank Wall - 10USG
Unuseable - 1USG per tank (this can be less in pre 1973 models so check the book!!)
Don't think anyone will bother going anywhere with less than 9USG useable!!!......or if one does, it won't be very far !!
The only thing I really use the gauges for is to detect a leak!! and even then the chances are that if it is a big leak, the tank will buckle and freeze the gauge!!
Regards,
DFC