“1 cubic meter of helium at sea level has a lift of about 1 kg”
Right, but that is at sea level. Denver is not at sea level, it is about 6,000 feet up. Not sure exactly what the capacity is at 6,000 ft, but I found this chart, that places it between 0.35 and 1.00/m3.
Altitude (m) Altitude (ft) Buoyancy (kg/m^3)
0 0 ~1
10,000 32,000 0.356055
20,000 66,000 0.076556
30,000 98,000 0.015852
35,000 115,000 0.00728742
Also, consider that if the craft was near its peak load when it was on the ground, it couldn’t possibly reach anywhere near the peak altitude of the unladen craft, it would top out at a much lower altitude and hover there before starting to sink.
Looks like it is an exponential decay so figure .5kg. IT would not be able to get very high.