I start by drawing a centreline down the back.
Then I draw 2 lines 11mm from each edge following the taper (i.e. parallel with the edge, not the centreline), then 2 more lines 11 mm down each side from each edge. These 4 lines define 2 chamfers, one on each side of the back of the neck. I then remove the corner wood to create the chamfers (I use rasps and spokeshaves).
I then draw a centreline down the face of each chamfer, then work on the 4 new corners (at each edge of the chamfer), using the spokeshave. By watching the distance from the neck centreline I drew at the start, and the new centrelines on the chamfer faces, I create a 4 new smaller chamfers, one on each edge. I start to let the chamfers meet in the middle of the back of the neck, and so the original centre line starts to disappear from the nut end first (because the neck is narrower there. The chamfers on the side, I let approach the fingerboard evenly.
By the time I have this done, I start measuring the neck thickness to get it down to what I like, and removing material straight down the centreline of the back of the neck. When the desired thickness is reached, of course, there is a flat spot right down the back. So, using a combo of spokeshave and a strip of coarse sandpaper (used in a similar manner to polishing shoes working side to side and moving slowly up and down the neck) I round out the neck to it's final shape.
Doing it this way keeps it symmetrical at all times and the resulting neck turns out great.