The fractional formula for locating back bodice horizontal guidelines from the nape of the neck is:
$$\text{Neck/Shoulder} = \frac{1}{8} \cdot \text{CB}, \quad \text{Shoulder Blade Line} = \frac{1}{4} \cdot \text{CB}, \quad \text{Armhole/Bust Line} = \frac{1}{2} \cdot \text{CB}$$