Real Black Powder Only. I can't speak for BP substitutes, I don't use them.
A square load means the same amount of powder and shot BY VOLUME. See the photo of the volumetric measure.
Pour proper amount of 1F, 1-1/2Fg or 2Fg Black Powder into primed case
The Nitro card wad is next, press it over the powder strongly (no air gap)
Then I used two fiber wads (rolled in melted bullet lube) pressed in firmly
One more Nitro Card just to take up more space
Pour shot over wad column
Press in the Over Shot Card, evenly and snug
I used Sodium Silicate to seal the thin Over Shot Card, however there are other products that work as well.
Caution Note: For Back Powder Loading Only.
My methods are what works for me. Others may do it a bit different.
Old Eynsford Black Powder delivers a bit more velocity and burns cleaner than Goex in my experience.
Conversion Chart for Black Powder Drams to Grains:
Drams - BP Grains
1 - 27.34
2 - 54.68
2.25 - 61.52
2.5 - 68.35
2.75 - 75.19
3 - 82.02
3.25 - 88.86
3.5 - 95.69
3.75 - 102.53
4 - 109.36

Why only black powder in brass cases?

No clue other than I've been told you shouldn't. I have found absolutely no data for smokeless in brass. As I said I have proceeded VERY carefully. I'm quite familiar with pressure signs in center fire rifle and pistol ammo and have watched carefully for any indication there might be the slightest issue.
In this case, I believe Bruce is referring to the fact that black powder is done by volume not weight. The same volume of smokeless as black would certainly be a serious issue. My suspicion is some of the old burst barrel horror stories involved charging a gun with the same volume of smokeless as one would use with black powder.
Thank you, Bruce, for the loading info. Is there a "best" among the black powder grades you mention for this load? Don't the different grades burn differently?

"Is there a "best" among the black powder grades you mention for this load? Don't the different grades burn differently?"
I'm using what I have on the shelf. Fg is probably better suited for 10 gauge, while FFg would be better for 12 gauge. The "Fg, FFg" designation refers to the size of the powder granulations, "Fg" has larger grains than "FFg". Pistols would use FFFg and for flash pan use in flintlocks, FFFFg is what is used. The smaller the granulations the faster it burns.
The "square" load for my 10 gauge is a common load for 12 gauge. I have no need to push the envelope considering the shotgun is 142 years old and I want to avoid destroying the stock or even testing it's limitations. I respect my Parkers and when they go to the next caregiver they won't have been abused by me.

Thanks for the tutoring on Black Powder 101, much appreciated. I'm with you on not beating up fine old guns as well as pounding on old shotgunners! Two or three snow goose hunts a year I feed an Ithaca Mag-10 or a Winchester Super-X a diet of rhino rollers but I stick with fairly light shot loads and aim for 1200 FPS with my SxS guns. I don't need anything more and sure don't need to damage a stock!

The pistol primers most these brass cases use wont reliably ignite smokeless powder in the roomy shotgun cases. You need brass cases that take a 209 primer if you want to use smokeless. Like the RMC and RCC brass shotshells, ideally brass shotshells should generate lower pressures than any plastic shotshell since they have more internal volume and no basewad.

Interesting. I am using Winchester large pistol primers and Longshot powder and am having no issue with ignition. I'm getting a nice clean burn (no powder un-burnt, very little powder residue in spent hull) in TOW 2 7/8" 10 ga. brass hulls w/ 1 1/4 oz. I DID initially load half a dozen that were too light and DID get a less than perfect burn, only about 900 FPS (I have a chrony that reads reliably shotshells). You could tell they were a bit lame when fired.

Its seems results vary significantly between powders and load methods using large pistol primers and brass shells with smokeless. One method ive read about to resolve potential ignition problems is to just take a fiber wad, punch a primer diameter hole in the center and glue it in the bottom of the case as a homemade base wad. I could see issues with that though like it coming loose and getting lodge in the bore and not realizing it. Im sure each powder has different characteristics when its comes to ideal pressures for a clean burn and how easy they are to ignite etc. I keep procrastinating on order brass shells, I wonder if I should order them milled for American 209s or Cheddites.

I'd be scared of the glue-in-basewad, sounds like a disaster waiting to happen. We just had a member injured and base wad separation is a suspect. The Winchester primer I use is for both standard and magnum pistol loads. I use them in .41 mag and .44 mag loads as well as 10MM and .45ACP so I had them on hand when I ordered hulls. They ignite 14 gr. of Blue Dot just fine, thought I'd try then in the brass, had good results. Now, the only powder I've tried them on in brass shotshells is Longshot in a 10 ga. Can't speak for any
other powder, gauge, or combo.
I see no reason not to order the hulls with 209s in mind.

I've shot smokeless powder through my 1907? damascus Ithaca 1-1/2 Flues 12ga in stainless steel hulls that took a pistol sized percussion cap on a nipple on the back of the shell. They worked great and got me a few spruce grouse. I haven't seen those steel cases since the early 80's. I do remember that they were pretty stout and took fiber wads if I remember right.