This is nearly correct. Rather than the weight of air volume the cylinder could contain, you want the weight of a volume of air equal in volume to the cylinder (including its walls) -- and you seem to have the wrong sign on the measured weight and the cylinder weight. Your equation says that the measured weight is equal to the buoyancy force, plus the weight of the cylinder, minus the weight of the helium, which would be weird.

(weight of helium) = (measured weight) + (buoyancy force, which is the weight of an equal volume of air at the appropriate pressure) - (weight of cylinder)

source: https://en.wikipedia.org/wiki/Buoyancy