I used to do the same thing you do, for the same reasons, I wanted everything as consistent as possible.
How do you account for weight differences caused by water content differences? Once you open that powder canister the water content (and even the nitrogen content) starts changing.
I left some Power Pro 2000 MR exposed to air for three weeks and measured the weight of the volumetric charge. It was "heavy" one grain (which is a lot for a 223 load). Obviously the powder had to gain water content, even though the total energy content of that volumetric charge is exactly the same as the powder stored in a supposedly dry canister.
Machine guns are awesome until you have to carry one.