For what its worth, I think water pumping generates much more effective expansion force at lower vacuum levels. If you are pumping and not making gains, try water pumping at the same vacuum level and see if you start to see progress.
The problem with the higher vacuum levels with air is you really can burst some blood vessels. If you are going to experiment with higher levels, go up 0.5 in hg at a time, do it for 30 seconds then pull out and see if you are getting red spots. If you are getting red spots, don’t go any higher. The red spots are the small capillaries bursting. They are the first to go, if you go higher, the bigger vessels will go next. I think its foolish to go higher if you are getting red spots.
On the other hand, if you up the vacuum and you aren’t getting red spots, then its probably safer to go up another 0.5 in hg and again test for spotting. Before you go higher, make sure you start to use the time you intend to use, to make sure they aren’t failing after a few minutes.
Some guys blood vessels are very strong and others not. I get major spotting at 5 in hg, but 4 is fine. 4.5 in hg is fine too, but if I jelque the tube even a little, I start getting spots. So there is a definite threshold for each of us where the blood vessel walls start to fail.
Remember, a major injury will set you back until it heals, so don’t be in such a rush that you end up taking far more time to recover from injuries.