As much as it's important to be familiar what kind of shape your breasts are or whether they're Full On Top (FOT) or Full On Bottom (FOB) or whether they have wide roots or narrow roots (or tall or short roots), the best place to start for getting the right fit is to Swoop and Scoop. Reach into the cup and bring forward all that breast tissue that is hanging out under the armpit. Those of us who've been wearing incorrectly fitting bras for years...hell, decades even, may have more than others; same if you're a bit heavier than say someone who may be in a 32 band. Bring forward all that tissue. I have to hang on to the outer edge of my underwire to really get a good swoop and scoop but, let me tell you. It makes all the difference.
Here's me before Swoop and Scoop (in the bra that has been my "go to" bra for about a year now):
Now, here's after I've Swooped and Scooped:
When you've got a bra on, Swoop and Scoop. Does it still fit?