Usually low pressure tanks are in the 200 psi to 500 psi range. High pressure tanks are in the 650 psi - 1600 psi range.
To run an Automag you're going to want to have 800 psi at the absolute minimum, and 850 psi would be more reliable. Higher pressures (950 or 1100 on a Ninja SHP, or even higher with a custom Ninja regulator, or some other adjustable regulator) make the recharge faster, and allow for a faster ROF when RT'ing the modern valves.
I've heard of some Automag valves operating as low as 650 psi output, but not reliably.