Thanks for the brilliant tip about floating canisters to gauge the amount of gas left with precision. So far, I've only done the hand shaking test (subjectively judging the weight of a partly filled canister versus a full one, and getting a feel of how much liquid gas there is sloshing around in there). Looking forward to trying out your method.

I have the same objections to gas canisters as I have to primary batteries: They seem to be either full (in which case I am reluctant to break the seal and use them), or almost all empty (leaving me in the dark/cold after 5 minutes or so).

If I carry a full canister I can resort to only carry one. If I insist on using up my half-filled canister I have to bring two, because I can't really trust the half empty to work. As a result, there is a collection of half-filled canisters in my shed...