Doesn't matter much - the Ion will handle low to high pressure. You have an adjustable bottle reg? If so maybe 400-500. I run an 850 preset and it's as sweet as a nut
u shouldnt have to, The gun will take any input you can throw at it between 350 and 850, the inline LPR (Front Foregrip) will reduce HPR & LPR to its operating pressure, since the guage is after this reg, thenguage wont tell you your input pressure.