It should be level all the way around. If it arches up your back, the band is too big. You should be able to fit two fingers underneath it.
When you put on a bra, lean forward and use your hands to pull all your breast tissue from the sides into the cups. This ensures you’re actually sitting in the bra correctly before you judge the fit. how to buy correct bra size
The center piece between the cups should sit flat against your breastbone. If it’s hovering, the cups are likely too small. It should be level all the way around
Your breasts should fill the cups without "quad-boobing" (spilling over the top) or gaping (extra space at the top). how to buy correct bra size