I have found to hard to load will cause flyers, and to easy will shoot all over the target, I like to keep the different sabots on hand and try them with the breach plug out at home before I take it to the range.
It sounds like either the regular harvester the MMP HPH24 or the Crush Rib is what you need.
My TC"s like the MMP HPH24,except the Triumph it likes the Crush Rib, all the Knights like the MMP12, I use PRB only ib my home made Hawken and the under hammer original likes MMP12. Lee