As you've figured out, there's no magic formula for determining breast cup size after breast augmentation. There are many reasons for this:
1) Your anatomy. The width and height of your breast, the quality of the skin, the width of your chest wall, and the amount of droop all impact final breast size. If you have droopy, wide breasts with a wide chest wall, a 340 cc implant will not project as much as it would in a a narrower, non-droopy breast. Less projection will often times correlate with a smaller increase in cup size.
2) Your perception of breast size. Different women perceive their cup size in different ways. Whereas one woman may think she has an A cup, another may call herself a B cup. The difference often depends on how tightly they wear their bras, the types of bras they wear, and how they classify their chest wall diameter.
3) Differences in cup sizes between manufacturers. A D-cup at one store may the same thing as a C cup somewhere else. Most women have figured this out, and often times will tell me that they want to be a "full B/small C" or a "full C/small D". To me, that means they know their bra size will vary depending on the store from which they purchase their bras.
Your best best is consultation with a board-certified plastic surgeon. After a detailed examination, he or she should be able to give you a reasonable estimate on whether a 340 cc implant will allow you to achieve your goals.
Hope this helps. Best of luck.