I think I'd try 2F, Bob, starting at 60gr.(& appropriate wads) and go from there. 60 of 2F might give similar speeds to 70 or even 75gr. 1F. I suspect 2F might get the speed up over 700fps, maybe even 750fps, without increasing recoil to an uncomfortable level.
The long barreled ctg. guns do well with 1F, but I'm sure the results might be better with 2F. It should be a lot more efficient in the shorter barrel lengths.