Well I'm not a ballistician but black powder burns at a much more consistent rate than does smokless under varying conditions, confined or un-confined. I don't think varying your components is going to cause you any problem with black unless you use something which doesn't seal & lose your pressure. An equal volume of black to 1 1/8oz shot is a comparatively mild load of a bit less than 3 drams.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra