I do the same math for long trips (I took my previous Skymaster, a '69 337D, from Miami to Boise, Idaho and back), namely 5 hours of flight and down for more fuel. That uses about 103 - 105 gal (considering take-off and climb consumption), leaving about 1 hour of reserves. It's critical, of course, that your tanks are filled to the brim (slowly, with
you inspecting) and that you know from recent prior flights that there are no leaks and that fuel consumption has been as expected. On a ferry flight of an unknown aircraft, I don't know if the last two assumptions can be made, so I would err on the side of caution -- I personally would not make a 5 hour trip over water.
Notice that I don't say anything about relying on the standard fuel gauges, given their propensity with age to be unreliable. If the aircraft has a digital analyzer with fuel consumption, that takes some of the guesswork out, if you know that it's been working well.
One technique that has been employed by some to get advance notice of remaining fuel near the end of a long trip is to cross-feed one of the engines so both engines run off a single tank for 30 minutes in the middle of the flight. When you go back to normal operation, one tank will have 10 gallons less than it should and the other 10 gallons more (if this is unclear, see the Fuel Supply Management page at
www.SkymasterUS.com ). So, if you are descending after the 5 hours and both engines are running, you know for a fact that the fuller tank has not 10 but 20 gallons left, because a) the emptier tank still has fuel after you "stole" 10 gallons from it, so there must still be 10 gallons of the
original fuel in the fuller tank, plus b) the fuller tank has an
additional 10 gallons, 5 that came from the other tank as excess fuel and 5 because no fuel was drained from that tank during the 30 minutes of cross-feeding.
One must be careful, long before you get low, to get back to a situation where there is ample fuel on
both tanks, so there is no risk of starving one engine while low and in the landing pattern. This is usually done by cross-feeding the opposite engine for 20 - 30 minutes while you are still high, to equalize fuel. One good rule of thumb is that a tank which is being unused for a period of X minutes, because its engine has been cross-fed and is drawing from the other tank, will have after you return to normal (non-cross-fed) operations fuel for about X minutes of engine operation. So even if the emptier tank was about to run dry when you started the 2nd or opposite cross-feeding to equalize fuel, if you cross-feed for 20 minutes, it will have fuel to sustain the engine for 20 minutes after you end the cross-feeding.
Obviously, this is complicated and
not recommended, but familiarization with these principles in the reference cited above might help you out of a bind on long-range operations. Especially with an unfamiliar aircraft.
Ernie