6 seems to be plenty for potentials, and 4 for for stat-gains. I almost never fail to gain a stat increase (~99% success) this way, regardless of the B/U/C status of the Morgia. Mareilon, on the other hand, I just play by ear. I'm not really very consistent with that (understandable though, considering most of my chars are elves and they don't really need much moss...)