Filling the cell completely won't work because as helium is released by a control valve, the strat will lose lifting power and fall.
Using mistressphaedra's math, to get to 65,000 feet, for every 40,000 cubic feet of helium it will lift 142 pounds (40000 * .064/18).
Once our payload weight is known, we add that to the weight of the strat for a total weight. Total weight * 18/.064 gets us back to the minimum cubic feet of space for helium it will take to get to 65,000 feet. The actual helium will be 1/18 of that number at sea level.