BWD and implant size, what parameters do you use to guide your patients and why?

Thank you for the good question.  
 
There are no hard and fast rules when it comes to diameter of breast implant versus diameter of breast (band width). In other words, these measurements are useful but are only one of the many factors come into play when it comes to selection of appropriate breast implant size/profile.  There is definitely some leeway when it comes to measurements and breast implant options (breast surgery is an art as well as a science).Every patient undergoing breast augmentation surgery should understand that their anatomic starting point will limit the outcome of breast augmentation surgery. The use of a larger breast implant (than what is safely "allowed" by the patient's anatomy) can be problematic;  potential problems include breast implant displacement/malpostion issues ( such as bottoming out, lateral displacement, symmastia...)  and/or significant breast implant rippling/palpability.
Other factors involved include surgeon dependent factors: for example, careful breast implant pocket dissection and careful selection of breast implant size/profile.
Ultimately, careful selection of plastic surgeon and  careful communication of your goals (in my practice I prefer the use of goal pictures, direct examination/communication in front of a full-length mirror, in bra sizers, and computer imaging) as well as careful measurements (dimensional planning) will be critical.
 
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 are 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. For example, I have found that the use of know words such as “natural” or "C or D cup” etc means different things to different people and therefore prove unhelpful.
Also, as you know, cup size varies depending on him who makes the bra; therefore, discussing desired cup size may also be inaccurate. Again, the use of computer imaging has been very helpful during the communication process, in our practice.
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.   I prefer to have a wide selection of implants (all profiles) available in the operating room. I use intraoperative sizers and place the patient in the upright position to evaluate breast size. Use of these sizers also allow me to select the breast implant profile (low, moderate, moderate plus, high-profile) that would most likely achieve the patient's goals. The patient's goal pictures are hanging on the wall, and allow for direct comparison. I have found that this system is very helpful in improving the chances of achieving the patient's goals as consistently as possible.
I hope this (and the attached link/video, dedicated to breast augmentation surgery concerns) helps. Best wishes for an outcome that you will be very pleased with. 
 

and I am not handcuffed by your natural breast width as some take the rule of not exceeding your natural width to an extreme.  Width of the implant is used to help diminish the distance between the mounds (you don't want symmastia) and provide side boob.  This is done within the guidelines of what my patients want as not everyone likes side boob but most do want less space between the mounds.  So implants are selected accordingly to what the patient wants to achieve.  In your case, I would need to know how much space is between your 14cm wide breasts to determine just how wide an implant you could take.