Min. operating pressure

800 psi is about normal output for CO2, you should be fine setting it at 800 psi. i'm not sure how much lower you can go than that, though...

Just today I got an old Model 98 (non Custom) operating with an Empire regulator.  I got 310fps with the first shot.  Keep in mind the Empire regulator claims a maximum operating pressure of 600psi.  I was a little worried that it wouldn't work right, but it seems to work great.  I am not sure how much lower I can go and still get it to work.  But I am still messing with springs to see the lowest I can go and still reach appropriate field velocities.

I only fired about 15 shots total, none in real rapid succession.  I just wanted to see if the setup (mainly the reg) was functioning correctly, and could actually reach field speeds.

I used the heaviest spring at first to find 310, then I went down a spring to the medium spring.  It still went over 300.  I then started messing with the regulator turning down the pressure a bit.  I experienced a bit of fluctuation in velocity, but I believe that is mostly because I only took a few shots, and haven't broken in any of the parts I have on it.

I need to get my own chrono so I can do this at home, the guys up there kept busting my balls because I was working on a Tippmann and not a >$600 electro of some sort.

Keep in mind that I took some steps to help lower pressure work for me.  I made modifications to the vertical adapter:  I drilled a second hole in the ASA part of it(also made the other hole a bit larger), and I opened up the hole on the other end with a drill (if you look inside you can see it is partially blocked).  I also lightened the pressure on the valve spring a little bit by putting in a shorter brass end peice (from an older gun).  I should have stayed at the chrono and tried progressively lower springs until I could no longer reach 300fps, but... I didn't.