There is a lot more variation in pressure with blackpowder than most realize. It is not enough to say Fg is slower than FFFg. Swiss Fg could produce higher pressure than Goex FFg. Swiss "120 gr" could actually weigh 125 grains, and Goex "120 gr" could weigh 115 grains. When people talk blackpowder, they are talking volume as thrown by a powder measure.
Anyway, I found a chart of low pressure 10 gauge loads. You can see Goex Old Eynsford 1 1/2 Fg is a higher pressure than Goex (standard red can) 1 Fg, all else equal. From what I can tell, the old traditional blackpowder cartridge 10 gauge loads were 1 1/2 or 1 5/8 oz loads with 4 or 4 1/2 drams of powder. There were plenty of variations, but that seems to be a popular area. 4 drams is about 110 gr, 4 1/2 is about 120 gr.
In the muzzleloader world, this would be called a square load. A powder measure set to 120 gr would drop about 120 grains of powder, and ballpark 1 1/2 to 1 5/8 oz of shot. This is a good and simple way to load muzzleloaders, as you only have to have one measure, or if adjustable, you don't have to mess with it. 117 gr powder and 1 1/4 oz of shot like listed in the chart below is just silly. I bet that load patterns like garbage. I like 100 gr powder, and 100 gr volume shot in my new shotgun, and I plan on using 100 gr powder and 1 5/8 oz #6 shot for turkey this year. Muzzleloaders made before about the civil war were all cylinder bore. Choke wasn't invented until around that time. Let me tell you, trying to get a cylinder bore to pattern for turkeys is plenty of work, but it feels good when you get there. Load changes make huge differences in a cylinder bore, where as a choked barrel is much more forgiving. What almost everyone across the board finds, choke or not, is that to get better patterns you use more shot by volume, than powder. 120 gr of powder and 1 5/8 of shot doesn't pattern great for me, or most anyone else. Reduce the powder to 100 gr, and patterns go from "Eh, wish it were better", to very good. Changes in your loads do not have a huge effect on pressure like they do with smokeless. That is the beauty of blackpowder. The stuff is as idiot proof as you will ever get.
I'd say keep powder in the 80 to 110 grain range, and see how much shot you need. If your barrels are choked full and fuller, you might be surprised by the patterns only 1 3/8 oz do at 30 yards. If trying to push the range out a bit, 35 or 40 yards, then you will likely need to go up more.