Where should the mass bias be placed to negate the asymmetry of the core, and make the ball roll most like a symmetric core ball?

Move the MB into the thumb or left <right handed>--that will help get it close to symmetrical. 

For the intermediate diff it depends on how the asymmetry was created. Putting the mb in the thumbhole could greatly increase or decrease depending on if it was created by an offset or if it was created something sticking out from the core