My wife measured my back interscye distance as 16" - half is 8". The distance in the draft is 1/6 chest + 1-1/2". With my 40" chest this works out to 8.17, or a little more than 8-1/8". Fitting shows this is already too wide, so I don't think I want to add ease.
Well what I mean is that there is always ease in a shirt back, it is never just your body measurement.
So your base body measurement plus ease. Usually the amount of ease depends on how close fitting the shirt is overall. This draft has 2" ease on the half, so think about where they are distributing the ease.
Just to confirm where that measurement is taken, it is mid upper back, from where the arm joins the torso to the other side in the same place.
Since by the draft calculations you end up with a back width that is a bit too wide at 8 1/8" it goes to reason that your body measurement cross back of 16" or 8" on the half is incorrect.
Or, the chest measurement itself is incorrect(too small) but lets assume the chest measurement is right.
How to judge if the neckline is correct? Well, that is hard to describe. Once you sew a collar on, it becomes more obvious. Lets say that yes, the finished seam line should touch your neck at the sides and sit higher in the front, than where it is now. Think about the position of a dress shirt collar, how and where it sits on your neck.
I think it will improve things by using a different formula for the neck.