Start with a garment you own
Lay a similar garment flat, smooth the fabric without stretching it and use the same unit for each measurement. Record the points you measured between. A flat garment width and a measurement around the body are different measurements.
Where a dress sits on the shoulder
For a cowl or slip neckline, start with a dress whose neckline sits where you like it. Measure from the top of its shoulder strap to the lowest point of the neckline while it lies flat. Compare strap placement in the product photos as well as the listed bust measurement.
If the straps adjust, set both to a comfortable length before judging the hem. Raising the neckline raises the hem, too. Check both in a mirror with the shoes you plan to wear.