What size will a 300cc implant with a breast lift take me too with a Already C cup?

Thank you for the question.   
There is no accurate/reliable/predictable correlation between size/profile breast implant utilized and cup size achieved. For example, I do not think that the generalizations of "200 cc equates to 1 cup" hold any validity or are helpful in any specific or practical way.  In my practice, I ask patients not to communicate their goals, or evaluate the outcome of the procedure performed, based on cup size references. Generally speaking, patients undergoing the breast augmentation/lifting combination operation should understand that it is significantly more complex than breast augmentation surgery only and that it is associated with a significantly higher risk of complications and need for additional surgery. 
Also, patients undergoing the combination breast augmentation/lifting surgery should understand that there are limits as to how large of an outcome can be achieved, given that breast lifting surgery does involve breast skin removal. On the one hand, breast augmentation surgery is expanding the breast skin “envelope” while breast lifting is (by definition) tightening up the breast skin envelope. These 2 forces are counteracting each other. 
Therefore, it becomes important to remove the appropriate amount of breast skin and to use the appropriate size/profile of breast implants to balance these 2 forces appropriately and to allow for achievement of the patient's goals while minimizing risks of complications. 
In my practice, "tailor tacking" is an important part of the mastopexy and mastopexy/augmentation procedure. It involves TEMPORARY  closure of the skin (for example with temporary skin staples)  to double check the amount of skin that is to be excised during the procedure. This maneuver is done to ensure that enough skin, but not too much, is removed. On the one hand, it is important that enough skin be removed to achieve the aesthetic breast lifting results the patient is looking for; on the other hand, is important not to move too much skin creating tension upon closure and potential wide scars.
The tailor tacking technique is very important when performing breast augmentation/mastopexy procedures. Again, this technique allows for a double check of the skin excision prior to the actual excision being carried out. In this procedure (breast augmentation/mastopexy)  if too much skin is removed it will limit the size of implant that can be used and/or the surgeon may experience tension upon  closure of the skin resulting in  wide scars, wounds healing problems, and/or implant exposure.
Also, generally speaking, the best online advice I can give to ladies who are considering breast augmentation/lifting surgery (regarding breast implant size/profile selection) is:
1. Concentrate on choosing your plastic surgeon carefully. Concentrate on appropriate training, certification, and the ability of the plastic surgeon to achieve the results you are looking for. Ask to see lots of examples of his/her work. 
2. Again, have a full discussion and communication regarding your desired goals with your plastic surgeon. This communication will be critical in determining which operation and/or breast implant size/type/profile will most likely help achieve your goals.
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 "D 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. The use of computer imaging may be very helpful during this communication phase.
3. Once you feel you have communicated your goals clearly, allow your plastic surgeon to use his/her years of experience/judgment to choose the breast implant size/profile that will best meet your goals. Again, in my practice, this decision is usually made during surgery. The use of temporary intraoperative sizers with the patient in the upright position makes selection of the best breast implant size/profile relatively easy.
Best wishes.

Hello,There is no reliable or reproducible way to equate bra cup sizes and breast implant size, and this is true for many reasons including subjectivity of how a bra fits and non-standardization of the bra garment industry.  I know this comes as a surprise given so much talk about it, but if you really look carefully at women who used bra cups as a goal and who posted after surgery, most of the time they are complaining that their desired cup size was not achieved, either too big or too small. Additionally, some surgeons like to regurgitate completely false equivalents between bra cups and implant volumes, for example '200 cc equals one cup size'.  I urge you very strongly to give up using bra cups as a measure for your aesthetic goals. Instead go visit a few ABPS certified/ASAPS member surgeons who will size you in-office using an unpadded bra and sizers, putting you in front of a mirror with a few of your favorite tops on.  This way you will get a real time view of how you will look.  Although not as high tech as a '3D Vectra', it is more effective and fool proof. Best of luck!

Yes, you’ll be a D-cup... but is it just size you want, or beauty?  Beyond choice of implant size, multiple other considerations determine how your breasts flow after such a complex procedure.