I think a lot of depends on the size of your ribcage more than any other factor. I have a 30.5 inch band, so even if I was a B cup, I'd still still look perky. As it is, I'm a 32D and they don't seem stop growing, while my ribcage continues to shrink. Proper AAs and suppression of T, not only free and total, but also of it's ugly cousin DHT, is essential. You can tell when the dose is right when your facial hair growth slows dramatically. I know I didn't take AAs, only E and P, for almost three months and my feminization grinded to halt and my boobs shrank (I think). Now, after only two weeks of AAs, I have lost 8 lbs. and my boobs are bigger than ever and are "doing" something again. They haven't hurt in awhile, but the areola is harder or something and sticks out more. Maybe I'm entering that whole mound on a mound stage or whatever.
But I also had boobs before transition, not man boobs, actual boobs, breast tissue and all. So, that prolly plays a major factor in my boobage. But I'd go to a 32B to have a more pretty face, one that isn't so andro. It's andro in a femmey, cute way, but still andro. I'd rather be prettyprettypretty.
I wouldn't count on the whole one cup size smaller, age, or anything. It's all genetics and luck. And in the end of you'e not happy save the $3K and get BA. Heck, I want BA cause even though I'm busty, I have to use duct tape if I want dramatic cleavage, which I do when I wants to turn my BF on lol