>"You may be aware that as a canister is used in an upright position, the propane will get used up very quickly (due to it's higher vapour pressure) and soon the canister will contain 99% butane. "
Not exactly. The reason oil refineries have those tall columns is that multi-step distillation gives more complete separation than a single-stage distillation. Yes, the first-used vapors will be more propane-rich and the last-used fuel will be somewhat propane depleted, but there's always some butane in the mix and some propane left behind. And the higher fraction of butane remaining – the lower the vapor pressure of the mix.
Best practices: Weigh your canister when new to the nearest gram. Write, "Full = 317 grams" on it. Ideally, also weigh a canister when totally depleted and write that as an empty weight on all your canisters of the same type and vintage. Then you quantify how much is left in each canister before a trip. Bring partial canisters on shorter, summer trips. And use newer canisters on winter trips, because, yes, the mix has more propane when fuller and you'll get better cold-weather performance.
Some of us have been known to refill canisters and jiggle with the propane/butane ratio rather a lot (those of us who NEVER have hot summers and want a canister to behave well to low temps), but that is a fairly advanced skill as there are far more ways to do it unsafely than you'd think.