This is probably one of the most important measurements one should take when making a shirt/top/dress but I have rarely seen it mentioned.
This will make fitting the shoulders so much easier as the shoulders are extremely important to fit and one of the areas that gives most people problems.
It's so easy to meaure yourself there so you don't need help from anyone. This measurement is not exactly at the shoulder tip, I would say it's about 1 inch towards the back.
Once you have the meaurement compare it to your pattern but since your pattern is probably only a half pattern, divide the meaurement in half (first add for ease and seam allowance).
To figure out how much ease you need find a shirt/top that fits you well and is similar in shape and fabric then measure the same area and see how much bigger it is than your measurement - that is your ease. Use that number to add.
This alone will help you tremendously in getting a great fit in the shoulder area.
Remember to always use your well fitting clothing in your closet as your guide to pattern alterations.
This will help guide you to a great fit...more to come on fitting.