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.
First up, we always say something like this, and it's not a bad attitude to have, however, I'm pretty sure I recall reading somewhere that the aus (where I'm flying) standards for the POH reauire that the takeoff / landing data is required to have a percentage 'fudge factor' - i.e. there's a margin built in that makes things realistic for the 'average' pilot, and aircraft. I'm not suggesting you use that margin, and a 'personal limits' buffer on top isn't a bad thing, but don't be too hasty to dismiss the POH.
Secondly, while not as comprehensive as IO540's method, I generally dip the tanks before *and after* each flight. That gives me a pretty good idea of fuel burn for the aircraft on an 'average' flight (though it lumps climb, descent etc all together). Longer flights will always do better, but guess what.. if anything the POH figures come out *pessimistic* - so I can plan the POH figures with confidence. Your mileage may vary!
@UV, I think you'll find IO540's '200 miles' was a little tounge in cheek! Otherwise, 500+ miles to dry, 400 with sensible reserves would seem ballpark.
@venkoj (original poster), I don't wish to be unkind - I suspect you're in training: the POH has all you need to know, you should probably have a chat with your instructor. If you're a 'fully fledged' pilot, some revision would be suggested.