I have viewed 4 times this video. As a biggner I have doubt. You had got measurements of ready shirt. You have cut front part as measurement of ready shirt. But you didn't get exact measurements of backpart solder and downpart of sleeve. So I have doubt that how it will be exactly as it is in sample shirt. You know after making it we have to sell in shop.
Do not take measurement of the back part. Always fold it double and cut it half an inch less than the front chest. Shoulder measurement will remain the same. Sleeve armhole down is done according to the armhole.