I don't think that December is half way through the production year.
Looking at production when (what date) would you place the half way point?
If you use just production days then Dearborn mid point is 28th of Feb but if you look at sales and actual production I would guess that half way was earlier than that date
NJ production days mid would be Feb 5th & San Jose production mid point would be Feb 9th. If the start and end dates in the Mustang Production Guide are correct - and my math
Of course we're working with just the month of December could be early or late in the month