I'm looking at the charge bar guide that came with the first MEC press I bought. I can't remember exactly when that was, but the guide is dated August 2, 1973 (! - where does the time go?).
mac1911 is essentially correct - the 12 gauge "O" bar is rated for 1-1/8 oz of shot and 18.5 gr of either Red Dot or Green Dot. (And also 20 gr of Dupont [now IMR] PB, 20 gr of 700-X, and 25.5 gr of SR 7625, all of which I believe are still on the market, as well as 20 gr of Alcan 120, which I believe is no longer available).
The best advice, which you've already received, is to weigh both the shot and powder that the bar actually drops, and use a reputable shotshell loading manual or online guide to ensure that the particular combination, and that includes the exact primer, hull, and wad type, is recommended as safe.