If I'm 40 H and have 1200cc removed, what size will I be?

It is impossible to say what size you will be after a breast reduction since every frame is different and every woman has a different idea about what bra size they wear.  There really is no standard of measurement.  1200cc is a good amount of tissue, but how that will translate to post-operative size is hard to say without examining you beforehand.

Congratulations on your decision to proceed with breast reduction surgery. It is one of the most patient pleasing operations we perform.  There is no direct correlation between the amount of tissue removed and the ultimate cup size that a patient will wear after breast reduction surgery.Before undergoing the breast reduction procedure it is very important to communicate your size goals with your surgeon. In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. I have found that the use of words such as “natural” or “C or D cup” or "proportionate" 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.Best wishes. 

After breast reduction the cup size can be unpredictable but often will correcspond to the chest size and width of the breast after reduction. Most will fit into a full C or a standard D cup, but if you have a very full figure as we would expect with a 1200gm reduction you might fit into a DD well after surgery.