To calculate the mass of the gas in the balloon, you will need to know:
1) V: The volume of the balloon.
2) A: The density of the ambient air (roughly 1.24 Kg/M^3)
3) B: The mass of the balloon itself (when deflated).
4) M: The mass as of the balloon as measured (positive or negative) as it floats in the atmosphere.
So your tare will be the volume of B-VA. That should be subtracted from the weight measurement.