I've played around quite a bit with different compression pressures on patched balls and have concluded that the best accuracy is obtained by having the ball just touch the powder with no (or very little compression).

I now seat the first ball very carefully until I feel it just touch the powder, then set the stop collar on the loading rod so all subsequent loads are seated to the same spot. Now this is with GOEX. I have no idea whether the same would be true with other powders. Also, you must remember to re-do the setting if you change powder loads.