I would think measuring the correct weight would be:
(weight of empty cylinder) + (weight of air volume that the cylinder could contain at atmospheric pressure) - (measured weight - positive or negative)
If you measured e.g. an empty cylinder, you'd end up with only the weight of air volume in cylinder at atmospheric pressure, which would be correct.
(weight of helium) = (measured weight) + (buoyancy force, which is the weight of an equal volume of air at the appropriate pressure) - (weight of cylinder)
> That's because the weight of the gas cylinder is relative and you got the point of reference wrong: The base weight is when it's a vacuum inside the cylinder, because otherwise it doesn't even displace the air which you don't want to measure for your reference weight, making it appear heavier. Whatever you add inside the gas cylinder now will make it heavier and the difference you get is the weight of the gas.