The answer is in the cartridge designation--.50-70.
Your load should be in the vicinity of 70 grains.
The powder to use would be either Goex 1F, or Swiss 1 1/2Fg.
NOTE: 2Fg black powder will HURT in that caliber, and it might be an overload.
3Fg WILL be an overload, and will more than likely damage or blow up your rifle. Do NOT use 3Fg in your rifle!
If you cast a bullet with grease grooves, lube it with SPG.
If you do not have grease grooves, you will need an overpowder wad, a grease cookie and a sealer wad to keep the lube from the bottom of the bullet.
You should compress the charge to ensure that there is NO airspace within the cartridge. Here's how I determined how much compression is needed:
Measure the length of the bullet from the crimping groove to the base.
Now, consider the length of the wads and grease cookie. Add that to the length of the bullet shank that you measured. For instance, I use a .030 King's fiber wad as both overpowder and sealer wads--that's .060. I use a .060 grease cookie. Added to the length of my bullet's shank, I came up with .60 below the case neck.
I load by first measuring my charge--120 grains of Goex 1Fg. I pour that into the case using a 36" brass drop tube.
I then insert a fiber wad into the mouth of the case, and use a compression plug in my expander die to compress SLOWLY to the desired amount. When I started the adjustment, I raised the case into the die, threaded in the compression plug and screwed it in until I felt it make firm contact with the overpowder wad. I then screwed in a bit at a time, and measured the distance from the case mouth with the depth gauge on my digital calipers, until I reached .60. The grease cookie goes in, then a second (sealer) wad. I then press it down with a dowel until it makes contact.
I'll then seat the bullet until I feel FIRM contact with the wad/cookie column.
I use a bit of crimp, just enough to iron out the bell in the case, and I'm finished.
Store your cartridges upside down in a cool place, so that the lube does not flow down in your powder charge.
Here's the finished product. These are for a .45-120 Sharps.