So now it's a simple matter of finding the middle of the neck, deciding on where the nut's face will be, and placing two dots 21mm from the center on this line.
This is the nut width.
For those who prefer the archaic system of measuring using the King's feet:
So the number it set to this.
And two lines are drawn up the neck from the nut width of 42mm, to the 19th fret width of 57.5mm. Even though I have 5 x 12mm pieces, the total width is 61.5mm, not 60mm - probably due to glue and board thickness variation.
This is what it looks like with a Telecaster neck Template sitting on it.
Those lines are very close ... I'm sure we can fit 4 strings in there!