Definitely a harder mix... 20/1, or Lyman #2. The size does sound a little big, too. Most like .458" best, but there's always an odd one. In any case, a sizing die is a cheap way to find out.
How new is the rifle? How smooth was the bore to begin with...I was careful to shoot some jacketed stuff through mine to burnish the bore (about 4 boxes, as I recall), cleaning with a good copper solvent every second or third shot (if copper fouling stays in the bore and lead fouling is deposited on top of it, you have a problem!). Seems to be pretty much OK. I use a lead solvent to clean after I get the powder fouling and grease out, which is easy.
