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Make sure you get the pot or pan holder if you have a regular jetboil. It's cheap and needed to hold the pan flat.
The most accurate way is to weigh the cannister. Obviously this requires a scale and some math and isn't to feasible in the backcountry. But is good to know and can give you a well educated guess as to what to expect from an individual cannister based on your use. Many of the cannisters have some sort of marking system on them, that will give a estimate of fuel left. Usually its a simple float test. This isnt real accurate, but I use it when I am confident that the fuel will exceed my use requirements. Another thing to consider if packing extra canisters is refilling them, g works makes a product that will top off your cannisters so you can always have a full one and leave those extra half empty deadweight at the truck.