If the rifle is a flintlock, don't waste your time. You would have to duplex the load.

Out of a percussion cap rifle, I shoot Triple Se7en all the time. And you're right, cut the charge 15% from normal. If you're shooting patch and ball and find the patches are burning up... put a wonder wad between the powder and patch and it will solve the problem.

Triple Se7en 2f will work just fine.