So you want theory?
Well, you want the CoG to be far enough forward to where the rifle isn't too whippy. However, this would change as the overal rifle weight increase do to accesories, stock choices, optics, etc. A heavier rifle would probably be ok with the CoG further back than a lighter rifle. Barrel length also plays a part here too. So not only is where the CoG is located, but also how much rotational intertia the system has.
Is that good enough theory for you?
In other words, it has to swing a certain way and it has to sit a certain way. More importantly than how it swings is how it STARTS to swing. Does the rifle want to roll? Should I move my support hand?
So you can't just move some stuff around and have the rifle handle well. You can help, but the bottom line is that some combination are just too heavy or too poorly balanced to work *juuussst riiiggghhhtttt*
Said Goldie Locks
*edit* I type(and think) like an idiot when I am really tired.