Hi mrcvs,
Use 3f of course.
To determine the practical volume for any given Cartridge using Black Powder, simply decide where the base of the Bullet will be once Crimped, and, have the uncompressed Powder Level be as much above that line, as subsequent compression will 'compress' TO that line.
For Revolver Cartridges, this would tend to be on to about 1/8th of an Inch or so - meaning, the uncompressed Powder will occupy a volume to about 1/8th of an inch Higher, than the Base of the Bullet is to be, once the Bullet is seated and Crimped, thuc compressing the Powder in the Bullet Seating procedure ( on one's Press of course ).
One has to sort of try a few and see - if the Bullet does not end up as deep as one was intending, once it is compressed well, then, for the next round, use a little less Powder.
If the bullet is deeper once compressed, than one had intended, use a little more Powder.
Once getting the right amount, pull the Bullet, and, weigh the Powder, and, or measure it's Volume and or modify a spent Case by cutting it down a little for it to be a Scoop one can then use, leveling it off with a Knife or however one likes, in order for it to give uniform results.
The old 'IDEAL' Powder Measures of Black Japanned Cast Iron, were intended for Black Powder or Smokeless, and they are very finely made and very finely adjustable and accurate.
That is what I use for BP Metallic Cartridge Loading.
Good compression is best...( meaning, one does somewhat 'grunt' on the Lever of the Press ).
One can also use a reduced Loading of Black Powder, and, add 'Cream of Wheat' to the top of it, to complete the volume which is to be compressed to end up where the Base of the Bullet is to be.
I compress using the Bullet itself, as I seat the Bullet in the Case, on my Loading Press.
A good Crimp of course, is best.
When getting back into Black Powder after a 30 odd year hiatus, and having bad memories of the mess and of using 'Crisco' over the Balls in Cap & Ball to avoid pecky 'Chain Fire' events, I decided to think about things rationally and in depth and from scratch.
I elected to try making 'Lube Wafers' using torn strips of regular Paper Towel, which I ran through a mixture of Molten Bees Wax and a little Olive Oil.
Once the strips are cool, I use a Gasket Punch 'Hole' Cutter over End Grain Pine or other Wood, to make appropriately sized 'Wafers' which are then about .035 thick or so.
I elect a diameter for the Wafer, which will 'cup' when put into a Cartridge Case ( Or Cap & Call Chamber ).
These worked perfectly, and, I could shoot any Revolver, Cap & Ball or Metallic Cartridge, more or less endlessly, with no 'binding' or fouling...no mess, no 'soot' on my Hands or Face, etc.
The tiny bit of Powder the Wafer displaces, is never missed or meaningful in the resulting FPS of the Bullet.
Swiss Powder is considered the best.
Your Revolver probably will prefer .450 to .455 Bullets, and, of pure Lead.
Do you have a Mold for this?