Scooping my irons

I seem to have the same issue as the OP.  I can't find the proper ball position.  I hit a lot of chunky shots and then when I try to correct it by moving the ball in the stance, I start to top everything and hit line drives.

It is very unlikely that ball position alone is affecting these kinds of issues.  Some people will go off of Henny Bogan's chart above and just alter their feet.  However, it probably has to do with the position of your body at impact though and not the position of the ball. 

Are you sure it's the ball position and not manipulation with your wrists and hands at impact?

Could also be an issue with posture.  I find I tend to top shots when I am too hunched over... on the way down my body unconsciously rises up to avoid hitting fat, and result is a topped or thin shot.

Percy Boomer talked a lot about being "braced upwards".  In other words even though you are tilted down at the hips to address the ball, and your feet are solid into the ground... you should feel an opposing brace in your shoulders going upwards.  This keeps the posture from dipping.

Ball position may play a factor but in my experience posture is a much more likely cause. If your posture is correct you will be able to bottom out your swing the same every time while keeping the swing arc at its maximum.

I have always been told that the 7 iron is the middle club, and each club longer moves the ball forward in your stance and with each club shorter moving it further back. 

The ball should never sit behind the heel of your rear foot and never further forward than the instep of your front foot. 

I generally play my 4i around the middle of my body (equal distance between my front and rear foot).  Each club shorter I play approx. 1 ball further back in my stance with my wedges being about 2 balls in front of my rear heel.  The driver I play off of my front heel.