Coverage is a design decision.
Specify the neckline width and depth, armhole shape, shoulder width, and strap position together. A lower armhole can change how much of an underlayer is visible, while a narrower shoulder changes the visual balance of the body. Front and back references are both important, especially for racerback or more open constructions.
Evaluate the sample standing and moving. Look at how the opening sits against the body and whether the garment behaves as intended when the wearer reaches or bends. A flat measurement is useful, but it does not replace seeing the actual coverage and proportion on the chosen fit reference.