None of the modalities to approximate final results from breast implants are 100% accurate; they merely facilitate the patient and the board certified PS concurring on the size and type of breast implant which will come the closest to reaching the patient's goal. My patients prefer to try on implant sizers (800cc in your case) under garments they typically wear (workout tops, bikini, tight sweater, low cut dress) to determine if they fit as expected and help allay some of their trepidation associated with this decision. I would recommend that you go with the size that boosts your self-esteem, and fits your anatomy. A very general estimate is that one cup size is 150 to 200 cc of volume, but realize that no direct correlation exists between the two. It's unlikely that you will notice a difference in increments less than 100. Implants of 600cc and higher will create more tissue thinning over time, have more potential for rippling, more potential for bottoming out, greater likelihood of breast lift revisions,