Isn't it that the higher the altitude you START at, the less the bubble expansion? You begin with less dense gas at 3000' then at 0', so the move from 3000' to 6000' has less effect than the move from 0' to 3000'.
I'm I thinking about this correct?
DAN agrees with you, Rick.
I asked DAN some extensive questions related to diving in New Mexico and crossing Raton Pass into Colorado. They said that the difference in altitude is less significant the higher you are starting from. A 3,000 foot increase in altitude is much more important when starting at sea level.