We used latex weather balloons from Where did we get them?. They are advertised by mass (350g, 1000g, 1200g) and each one has a different rated bursting diameter. Using the bursting diameter and using equations as pressure varies by altitude we were able to predict a bursting altitude with varying amounts of lifting gas. We were able to calculate the amount of lifting gas we needed by weighing our pods and using the buoyancy of the lifting gas in question (Hydrogen or Helium).
To minimize the amount of lifting gas we purchased (save money) and filled the balloon (increase altitude) we created formulas to find out how much lifting gas we needed for a given weight.? []