[Bruce Sexauer]

This is a sine/cosine related geometry problem. If only we had paid more attention in high school geometry! The angle of divergence between the strings and the fingerboard is (or should be) less than one degree. I suspect one could multiply the string tension (nominally 150 lb) by the sine/cosine (I don't know which) of the angle of divergence and get the exact figure for deflection at 90 degrees. Perhaps it is more convoluted that that, but I think I'm close. I have seen the actual math at some point in the past, but took away only the conclusion as the part that was useful to me, and the conclusion was as I stated earlier: about 4 pounds at the nut perpendicular to the fingerboard. This is mostly useful because it allows me to use my set up bench's built in jig to add four pounds of pressure to the nut end of the fingerboard while holding the body in stasis, and then lock the whole guitar in the position so that I can do fret work with the strings off but the neck in a position closely simulating where it would be at full tension.

I am guesstimating the 4 lb using my hand to generate enough pressure to bring the fret surface to straight (counteracting the properly adjusted truss rod). For all I really know it s 3.5 or perhaps 5 lbs., but not likely outside that to bring the surface level. So, seat of the pants, I accept the figure I read what may now be 40 years ago. One nice thing about physics, and its language (math), is that 40 years may make a difference in our understanding of it, but the reality itself stays the same. Apologies to Schrödinger and his cat, of course.