Measuring Shoulder Width
These measurements are to measure the distance between the low shoulder points-but are taken as a mostly straight line. These measurements do not curve around the shoulder, instead they stop when the tape reaches the straight line that extends the low shoulder point to the front and back. If it is helpful, imagine that the arm does not exist past the low shoulder point, and you are measuring edge to edge.
These images show measuring the front shoulder width. The tape is straight across the body just below the neck and connects the low shoulder point extensions. The tape does not curve around the arm. The tape is pulled taut (due to body shape it may look like it is loose along the top as it does on the model below, but the bottom of the measuring tape is tight against the body.)
These images show measuring the back shoulder width. A piece of string has been threaded through the starting side of the measuring tape so that it can be held in the hand when self-measuring. The tape is pulled taut along the body, starting at one shoulder point extension, and connecting the shoulder point extension on the opposite side. The tape does not curve around the arm. The measuring tape will make a slight curve to follow the shape of the body just below the neck, this allows the generator to take into account any curve at the base of the neck.