Sometimes there can be a lot of fiddling, especially without someone right there to help. The directions have been good. Sometimes reading what others say can help clear up confusion because they explain it different. Some directions, such as for a coat, a bit different, but close enough to figure out can help. If it adds more confusion then stay with the original lessons.
One method is to draw the pattern on paper and cut with enough room for adjustments. Pin this to center back. Someone else needs to do this. Pin on both sides of the shoulder so that the shoulder and back are sitting right. Another words, not pushing against the neck, nor standing away. And next is the front. Draw your new lines and that is your pattern. Now you can recalculate the pattern directions, If you want. In some perspectives this is similar to the string method mentioned above. Shirts and coats have different dynamics and achievement is persude differently. The stand is cut different as is the leaf for shaping. A coat collar is shaped with a hot heavy iron and water (steam). To wide seam allowances cause problems. I would aim for 1/4 inch. Larger will get in the way. The shoulder seam should end without going up the neck. Maybe half a stitch further.
Some tailors don't care about all the math because of all the variables, and for them fitting is faster, then the pattern is made. Some of these guys work by eye very quickly. A good education in pattern making is very handy, but some can grab ancient pattern methods with two-three measurements and make excellent garments. Different frames of mind can speed up the process. Beginners are tied to the pattern, because, that's all they got. Some experienced tailors look at the customer, see what they want, and the pattern doesn't matter much. Clothing is art.