I would shoot 3 1/2 to 4 drams with 1-1/8thoz, to MAYBE 1 1/4oz. shot.

ML's were 'usually' not loaded heavy, like the later ctg. BP shotguns.

In a non-choked guy, I would start at 3 1/2 drams (95gr.) 1F or 2F, then 3 thin overshot wads on the powder, then the shot (no cushion), then 1 thin overshot wad.

The idea of the overshot cards between the powder and shot, is they have little weight, inertia and are less likely to screw up the pattern. Guys on the ALR site came up with that one for their muzzleloaders to make good patterns with cylinder bores. The odd guy who actually worked at it, was able to get turkey head and neck killing patterns to 35/40 yards, pretty good for a 20 bore. Some simply overload their 20 bores with modern-type 10 bore loads of shot and 3 1/5 drams of powder, but rarely do better than 30 yards.

Heavy fiber wads tend to blow into the rear of the shot cloud by muzzle blast and cause the gun to shoot donuts.

This was evident in Greener's book with the high-speed shadow graph pictures, showing the difference in shot clouds leaving the muzzle with both choked and non-choked muzzles.

Too - you can use rolled post-it note pad slips for making shot-tubes. If at 20yards they go through the target paper like a slug, try patterning them further out, like 40-50-60 yards. You might be surprised.

As well, with the tubes, if well undersize, when pushing them against the overpowder wad(s), the paper may split(if weakened) which will also aid in shot dispersion.

Some guys got amazingly long range with their flint lock shotguns using partially slit bank coin rolls.

This might give you some ideas.

http://www.muzzleloadingshotguns.com/articles/eleypatentwirecartridges

I see the 10 bore load was 1 3/4oz shot in the Ely shot ctg.
Just in case you thought buffered loads were something new.

:"The paper wrapped wire basket was filled with lead shot mixed with bone dust. The bone dust acted as a buffer which kept the shot steady in the package and helped to prevent the shot deforming. Due to the significant difference in densities between lead and bone dust, the lead shot was most likely placed into the wire basket first and then covered with bone dust. By vibrating the cartridge, the bone dust would fill in the empty spaces in the shot, making a more solid shot package. Modern experience suggests that a light weight buffer can have a positive effect on pattern densities, particularly with larger sized shot."