Denny - you are fortunate to find a deep grooved TC.
First off, I'd crown it as described many times right here on ALR - if that isn't already done.
I'd start with a .490" RB of pure lead, .018" to .022" denim patching, spit, neetsfoot or mink oil for lube. Water based only for target shooting - not hunting.
I'd start at about 75.0gr. 3F and 2f and go up to 100gr. of each to find it's sweet spot.
I'd shoot off a bench, using a single bag supporting my left hand which holds the rifle as if shooting offhand.(right hand shooter) Hold the rifle in tightly to your shoulder which will be an easy 'hold' to duplicate each shot, just as if you were shooting offhand. Thus, the ball impact/grouping will give the same point of impact - most likely.
Do not let the gun bounce off the front bag as in modern BR shooting. This will give false elevation to the shots as well as magnifying any changes or variation in your hold, stock to shoulder fit, etc. Holding the rifle deliberately will give tighter, more consistant results.
I hold most rifles at the entry pipe or slightly back from there with my left hand gripping the forewood and barrel.