Just load it with the standard black powder load as if it were a Vaquero or Colt, 35 grains (or less with a filler) the Ruger will handle that. There is enough gas ring on the cylinder to let that gun go through a match without binding on the cylinder pin, an advantage over the Remington conversions.
The only thing to be aware of is the bp substitute Hogdon 777. It is about 15% stronger than black powder. The Ruger will take it, but you will be very aware of the fact you pulled the trigger.
If it is too much for you, back it off, though Hogdon says not to use a filler, and the bullet should touch the powder with no airspace. Tou may have to live with the heavy recoil of a full load with that powder. The powder makes for a good hunting load, but a full case of it is a bit much for a CAS match.
