Step 5. Underband
Most support in a bra comes from the underband. Fasten the hook on the last setting when fitting a new bra, as elastic will loosen over time, giving you room to tighten the back.
If the band rides up at the back, it is loose so go down a band size. When going down a band size, go up a cup size to compensate. Example 34B becomes 32C.
You should be able to fit two fingers underneath the band for a comfortable fit.