Bill Cumpiano is a very well respected guitar builder and has long been the "gp to" guy when problems occur and questions come up. He is the author of "Guitarmaking: and has this to say about fretboard relief:
GETTING RELIEF
"When comparing the perfectly straight, stretched string to the fretboard surface (the surface determined by the tops of all the frets), the difference shows up right in the middle as "relief": that is, what actually needs to be adjusted by tightening or loosening the rod’s nut. No relief, that is, the string pressing directly on the frets in the middle, betrays either of two incorrect situations: a perfectly straight fretboard surface, or a fretboard surface which is bent back. A back-bent fretboard results from either an over tightened truss-rod nut, or (heavens!) a permanently kinked neck-shaft.
What you want to see, however, is a minute amount of "relief," or clearance space, between the string and the fret right at the midpoint between the first and twelfth frets. If you can measure it, it should be:
Electric guitar: 1/64"
Acoustic guitar: 1/32" to 3/64"
Electric bass: 3/64"
But I don't measure it. Those dimensions have become so familiar to my eye, they're like old friends. The precise amount is not crucial, believe me. I just look at the space and image a paper's thickness in there (electric guitar=tissue paper; acoustic guitar=typing paper; bass=stiff card stock).
With the strings up to tension, loosen or tighten the truss-rod nut until you achieve the proper relief. On double-rod systems, such as those on Guild jumbo 12-string guitars or Rickenbacker electrics (assuming they’re operating as they should) always aim for a hair more relief on the bass side. Turn the truss-rod nut no more than one-eighth to one-quarter turn at a time. In between turns, coax the neck in the desired direction by pressing it over your knee – firmly, but gently – once or twice before rechecking."
Now remember he is talking about a guitar scale which is about half again as long as a tenor uke scale so his measurements will be greater than ours. Also ignore any mention of adjustments, since few, if any ukulele necks are adjustable.
Hopefully you've learned something about neck relief. If more manufacturers took the time to take into consideration that nylon strings vibrate in an arc when strummed, there would be a lot fewer questions about string buzzing on this forum.