A few calculations made on the flight of Pegasus II.  We target an ascent rate of 5 m/s, then work out the remaining calculations.  It is always a trade off between max altitude, mass of payload, ascent rate, and balloon size.  We must be careful to avoid a floater (neutral buoyancy) and ensuring the tethering is has enough tension, i.e., > 500 grams of residual lift.  We don’t measure the volume of Helium, rather the Neck Lift as the balloon fills.