I finally managed to get back to the neck project. Here's a table I made with measurements. The 2nd and third columns are measurement I took right off the x-rays. The fourth column are actual neck measurements. The last column is the calculated thickness between back of neck and the truss rod.
I set up the router with this MacGyver-style jig (Ikea shelves came in handy):
Here's a look at the back of the neck around the first fret:
And at the body end:
(Sorry I kind of missed with the focus there
I thought taking off roughly half to less-than half was being conservative, keeping in mind the that the radius of the neck actually decreases the depth (as Double E rightly observed). I actually took off 3.8, 3.4, and 2.2mm at the 1st, 7th, and 12th frets. I was very happy with how the router did the job. I still need to finish the carve and refinish the neck, but I think it's going to be much more comfortable to play. It already feels much different.