If I'm a size 18 cup size, will 435 cc bring me to a DD cup?

The best
cosmetic result in any particular breast augmentation patient depends on a
variety of factors, including: Your individual anatomy, realistic expectations,
a thorough discussion with the plastic surgeon about the options, and an
understanding of the pros and cons of any particular implant choice. Proper
sizing is not just about the number of cc’s. The thickness of your tissue,
breast dimensions which include the width,
height, and projection, as well as chest wall width all need to be considered
when choosing an implant. Trying on implant “sizers” of various shapes and
volumes while wearing a tight t-shirt, bra, or bathing sit at a preoperative
visit will help you and your surgeon choose the optimal implant.

There are no manufacturers' standards for cup sizing in the bra industry. The
cups of a 32 D and a 38 D are significantly different. Cup size varies from
manufacturer to manufacturer and even within styles from any particular
manufacturer.

Keep in mind that following the advice from a surgeon on this or any other
website who proposes to tell you exactly what to do without examining you,
physically feeling the tissue, assessing your desired outcome, taking a full
medical history, and discussing the pros and cons of each operative procedure
would not be in your best interest. I would suggest you find a plastic surgeon
certified by the American Board of Plastic Surgery and ideally a member of the
American Society for Aesthetic Plastic Surgery (ASAPS) that you trust and are
comfortable with. If you are seeking a plastic surgeon out of the United States, he or she should be a member of the International Society of Aesthetic Plastic Surgery (ISAPS). You should discuss your concerns with that surgeon in person.Robert Singer, M.D., FACS 
La Jolla, California

There is no accurate correlation between size of breast implant use and specific cup size achieved. As you know, cup sizes will vary from one bra manufacturer to another and probably should not be used as a reference point when it comes to communicating goals or judging outcomes.Generally speaking, the best online advice I can give to ladies who are considering  breast augmentation 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're looking for. Ask to see lots of examples of his/her work.
2. Have a full discussion and communication regarding your desired goals  with your plastic surgeon. This communication will be critical in determining  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  "C or D cup”  etc means different things to different people and therefore prove unhelpful.
 Again, cup size varies depending on who makes the bra; therefore, discussing desired cup  size may also be inaccurate.
 The use of computer imaging technology may also be very helpful during this communication process.

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.
I hope this (and the attached link) help.