Too tight signs
Painful red marks that last hours, restricted breathing, indentations into the back muscles. Sister-size up (band up, cup down).
Too loose signs
Band rides up, hooks slip, you wear the tightest hook on a new bra, straps do all the work. Sister-size down (band down, cup up).
Just right
Two fingers slide flat between band and back. The band stays horizontal at the same height as the front. No slipping, no digging.