I'm a bra fitter, by profession and passion! I work in mothercare where I fit no wired bras only. These have to fit properly as wires will make a badly fitting bra sit "right" , there is nowhere to hide without wires. I have also made my own bras so I understand the "mechanics"
To properly fit a bra you need to take measurements while wearing a well-fitting non padded bra.
To get your band size measure firmly under the bust and......
Add 4, if you need it, 1 more, your looking for an even number (did I mention I was a trainer)
To get your cup size measure loosely over the fullest part of the bust making sure the tape is level.
Cup size is the difference between band and cup measurements
No difference... A cup, plus 1 B, plus 2 C
So far so simple..... But
If your wearing too small a cup and spilling out you will get measurement of the cup your wearing,
Some people are "all back" and others " all bust" , I have a very good visual aid I use then training , but basicly if it doesn't look like a D cup it probably isn't.
style affect fit, colour affect fit, yes really! Putting a bit of lace on the bridge affect the fit. Power bands and cup seams really alter the fit! So don't be surprised if your sometimes different sizes even within bands.
Now hooks...... Bras are not designed to go baggy! You have multiple hooks because you can be either 31 or 32 inches under bust to be a 36 band so you have two hooks 1 inch apart, extra hooks are for days when you need an agustment, my pregnancy/nursing bras have 4 hooks but I use these to allow for a woman's changing body
Put a bra on properly don't "clip and swivel"
Lean forward and let your breasts drop into the cups and clip to where it is comfortable today and adjust the straps. Always take the stretch through the cup when adjusting the strap, don't just pull from the top of the cup.
If you need a well fitting G,H or J cup come to mothercare (other bra fitters are available!)
I wear a 36 DD or a 38 D if it has a lace underband, I never take my bra off when I come home anymore since I started wearing our non wired brasi just ignore the nursing clips
A well fitting bra is our right just be patient with a fitter, always try before you by and get a bra that fits now not when it's "gone through the wash"!