OT: Cubic Feet in a Compressed Gas Cylinder?

Totally off topic, but I though someone on the list might know how to figure
this.

I am looking to be able to calculate the number of cubic feet of gas at
standard atmospheric pressure that are in a cylinder at any given time. The
2 scenarios are:

If I know the rated capacity of a cylinder at a given pressure How do I
calculate the remaining cubic feet at a different pressure. As an example,
if I have a cylinder that is rated to hold 219 cubic feet at 2080 psi, how
many cubic feet does it hold at 500 psi or any other pressure?

If I know the internal volume of a cylinder how can I calculate the capacity
in cubic feet at 100 psi? As an example, if I have a cylinder that has a
calculated internal volume of 3.5 cubic feet, how much gas can it hold at
100 psi.

TIA,

Tim
[Denver, CO]