I'm assuming this is just an academic exercise to get a ball park idea of theoretical range before the engine stops - nobody in their right mind would try it in reality!
Endurance x IAS = still-air range
The trouble is that endurance figures from the POH are 'showroom' figures based on a new engine in a shiney new aircraft, probably using recommended mixture leaning techniques at the optimum power setting with a test pilot on board.
Personally, I fill it and allow a 1hour reserve - over-cautious perhaps, but fuel exhaustion is a significant cause of GA incidents and I wouldn't like to become a statistic.