How many grams should I remove from each breast to relieve my back pain? I want to be a 30 B/C.

 Unfortunately, your plastic surgeon ( and online consultants) will not be able to predict, with any degree of certainty, what cup size you will end up wearing after the breast reduction procedure. There is no specific correlation between amount of tissue removed and resulting cup sizes.
 Most patients  undergoing breast reduction surgery wish to achieve a enough of a reduction to help with their symptoms while remaining proportionate with the remainder of their torso. 
With the goal of improving communication with my patients (determining their goals in regards to breast size after breast reduction surgery) I find the use of photographs of “goal” pictures (and breasts that are too big or too small)  very helpful. I have found that the use of words such as “natural” or “C cup” etc. means different things to different people and therefore prove unhelpful.
Also, as you know, cup size varies depending on who makes the bra; therefore, discussing desired cup  size may also be inaccurate.
 
I hope this, and the attached link, helps.

Without an exam in person, it is difficult to say what is possible and approximately how many grams could be removed.  Good luck.

Please express yourself to your surgeon.  If you wish to be smaller, convey that wish.  While there is a limit to making someone too small due to anatomic limitations, your desires are what we surgeons wish to meet. Revisions for volume reduction are sometimes done with liposuction.  No new excision of skin may be possible.  Please see your surgeon for details.