One thing I would suggest is to take a Sharpie and write a number on each canister (1-6) Use the canisters up completely in sequence starting with canisters one and two. Don't use canister three until canister one is empty. Don't use canister four until canister two is empty, etc.
Ditto that suggestion! For all trips.
Also, weigh each canister before leaving and write the weight on the can. Weigh any partially full canisters when you return. The diff is how much was used, write that on the canister.