The high point shoulder or HPS point is used as the base for several vertical measurements. Refer to the hps documentation for information on how to locate this point.
The high point shoulder (HPS) to bust measurement runs from your HPS down to the line that forms your bust span.
To measure your HPS to bust, first localize your HPS point. Then measure straight down to the line that forms your chest circumference/bust span.
The point of this measurement is to locate the fullest part of your bust vertically on your torso.
If you measure yourself, you might try to hold your measuring tape up to your shoulder with the hand on the same side of your body as the shoulder you're measuring. So, for example, you could be holding the top of the tape up to your right shoulder with your right hand, while your left hand is holding the end. But that means you distort the length you are trying to measure by raising your arm on the same side. You might not think there is a difference, but all is connected in the world of physiology, and doing this could lead to an error of ~2.5cm (~1 inch) in your measurement.\ Simply switch your hands around, and you get a more accurate measurement.
For best results, get a second person to take your measurements (if possible).