Ideally, how much narrower should your implant be than your natural breast width diameter? (photo)

Nothing is more reassuring, than hearing from a RealSelf user who is thinking in terms of diameter rather than "cc." Patients often think in terms of cup size when considering augmentation. Unfortunately, devices are sized in terms of milliliters (cc) of volume. This can lead to some confusion when sizing. Additionally, it is important to remember that cup size itself is not standardized with variations from one manufacturer to another. Unfortunately, as many women can attest their cup size in an industry leader such as VS is not necessarily transferable to another brand.
Another point which is often under appreciated is that of anatomy and starting point. Any implant will add volume to the volume which is already present. The implant is additive. A particular volume will not necessarily confer the same cup size to different patients (often times it will not even confer the same cup size to different breasts in the same individual...remember they are "sisters" not "twins"). 
A general rule of thumb is that 125cc can represent somewhere between 1/2 to a full cup size increase. Smaller volume differentials (25-50cc) are typically less consequential representing a volume change of less than a shot glass. However, I have found these numbers, at least anecdotally, to be of little help. Patients often present with notions/goals which do not correlate with these sorts of sterile volumetric assessments.
When sizing patients, there are a number of useful tools including:
-3D imaging (has the added benefit of offering a volumetric analysis of the pre-operative breast)
-Breast sizers (rice bags)
-Goal photos
I also recommend that patients commit to a particular look rather than a cup size. Once a patient settles on a look that pleases them the overall cup size increase becomes less relevant.
With regards to your particular question, when considering 410 form stable shaped gels, the patient should aim to match or slightly exceed their planned pocket dimensions to ensure a snug fit. Undersizing can lead to malrotation. I tend to favor round gels for augmentation as I do not feel that the upper pole fill adds much in the setting of a patient with native breast tissue (as opposed to post-mastectomy patients with no native breast tissue). When using a round gel staying within 1 cm +/- of your measured with is advisable (with a preference towards "-" so as to avoid the dreaded lateral show and "fake" look).
As always, discuss your concerns with a board certified plastic surgeon (ABPS).

Thank you for the 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.
You are correct however: 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.  Remember (when it comes to breast implant pocket dissection and achievement of ideal breast implant cleavage) that each nipple/areola complex must be centered on each breast mound.
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.
***Given that your surgery is coming up soon, I would suggest that you spend additional time communicating your goals/concerns directly with your plastic surgeon, preferably prior to the date of surgery.
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. Again, in my practice, this decision is usually made during surgery, after the use of temporary intraoperative sizers.
I hope this (and the attached link/video, dedicated to breast augmentation surgery sizing concerns) helps. Best wishes for an outcome that you will be very pleased with.