The Across Shoulder measurement is usuallydefined as; Shoulder Tip to Centre Front Neck.
The image that would accompany it would be something like the introductory image on this page, showing the Centre Front Neck to be the pit of the neck which is the top of the Bodice Block. This measurement which is found in other patternmaking instructions (sometimes with a slightly different name) is usually shown in images as a straight line, or sometimes with a slight tilt upwards to the neck.
This caused me a lot of grief. I couldn't understand why my Bodice Front seemed wide in the neck with the shoulder slope too slanted. I was using my measurements, and I checked them again and again.
The Across Shoulder sets the width of the block, and if that measurement is wrong you will get the following problems: neck too wide, incorrect shoulder slope, and the sleeve edge too far off the shoulder. See the image titled: Incorrect Across Shoulder for an example.
I eventually figured out that measuring from the Shoulder Tip to any part of my neck wasn't working for me because of my body shape. The measurement from the Shoulder Tip to the Pit of my Neck was on a downward slope and ended up being 1.5cm longer than my actual Across Shoulder width. Measuring across to my neck didn't work out any better because my neck is on a forward angle, and so that also resulted in an incorrect measurement.
I needed to measure from my Centre Front, quite a distance down from my neck, to what would be the shoulder line. See the example in the following image. Compare the standard way to measure (Across Shoulder A), to how I had to measure (Across Shoulder B).
Be careful if....
If the measurement from your shoulder tip to the little dip in your neck is on a very downward slope (and this may apply to very upwards slopes??) and you measure the standard way, you end up with a much larger neck and the sleeve edge too far over the shoulder. In this case measure as I did - see the third image on the page marked Across Shoulder (B).