I too have the same barrel and I get good results with 70 gr of Pyrodex P and excellent results with 80 grs pushing a 200 gr XTP(10 mm) bullet in a Harvester light blue EZ load sabot.
Excellent accuracy, a joy to shoot, and a good deer stopper.
Your Goex FFFg should give you similar results.
I would say start at 70 gr and work you way up in 10 gr increments until you find your barrel's sweet spot.