The best-fitting cups are smooth and seamless without gaps or spilling. Your cups shouldn't leave space between you and your bra.
Spilling over? If your cups don't fully contain you, they may be too small.
Cups don't fill out? A smaller cup size could be the answer.
Fasten your band on the middle hook so it sits level on your back, feeling snug but comfy.
Digging in? Try to loosen your hook. If it's still too tight, you'll need a larger band and maybe a smaller cup size. Perhaps re-measure yourself for the most up to date size!
Is your bra riding up? Try a tighter hook or go down a band size. You may even need to go up a cup size.