Looking at my 8 ounce (or near) fuel canisters:
I went through a rather lengthy exercise a couple of days ago and found that different manufacturers vary a little on the can weight. Five ounces was near average for my collection. I found one completely empty (Coleman brand)at 5.2 ounces so I am using that as the tare weight. So when I subtract the tare wt. from the gross wt. I get a conservative net wt.(the fuel).
I mostly use an older MSR pocket rocket and figure on (nearly) boiling 16 ounces of cold water twice a day and so one ounce of fuel per day seems to be more than enough.

My biggest problem seems to be having a lot of partials. If you use several partials in the field you carry that extra canister weight of course. I know there are ways to refill them, but it sounds like too much of a hassle to me.