[Lotus]

I think you're at stage 3, follow sfem's (still the best way) to calculate actual cup size. From sfem's guide you can easily track breast growth. For reference my cup size is 15 inches (an H cup), when I started using sfem's guide I was 9.5 inches, 

Stage 3: Breast tissue palpable outside areola; no areolar development

Cup Size
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Measure the size of your boob (yes, just one boob). Place the tape horizontally and measure from the edge of your sideboob where it meets the chest wall under and in front of your armpit, to where the boob flesh ends at the chest wall near the middle of your chest in front. The ratio of band size to boob size gives a cup size as follows:
Ratio | Cup Size
  4.6  |    A
  4.3  |    B
  4.1  |    C
  3.9  |    D
e.g. I measure 9" of boobage. On my 38" band, my ratio is 4.22 (38/9) and my proposed chart suggests my cup will be between B and C, closer to B than C. My best fits are B cup bras, although some C cup bras fit me too. I can't get into an A anymore. The ratios above are rounded to a single digit of precision so your mileage may vary.

Hope this helps.