Finally got my O2 from welder supply, does anyone know what to set the gauge at for a psi gauge? The gauge I have is a Smith medium duty regulator series 30-100-540.
I cant use the tank until Im sure what to set the gauge to. I want to set it at 15 LPM
Thanks in advance.

I take deep breathes and long exhales so the bag fills up almost full even at 10 LPM, but I almost empty on the next inhale. 
Somebody on here said about clearing the lungs as much as possible before inhaling the next breathe.
With this last cycle I never sat on the tank longer than 10 minutes and the direct pain was killed within 5 for 99% of my hits.

As others said, you can't convert psi (pressure) to lpm (flow).
You can convert gpm to lpm or PSI to kilograms per squared centimeter or meter.  Maybe that's what you're thinking.  You're regulator should do 2 things.
Tell you the pressure in the tank and the output flow that goes into your mask.  I will look up that regulator you have to see what it says.