Sam, I searched around and found this:
Since the amplitude of the vibrating string depends upon the type
and tension of strings used and the "attack" used to displace the
strings, the amount of neck relief must also depend upon these same
factors as well as the individual preferences of the player. While
there is no one universally correct setting for neck relief which
accommodates all the variations of these factors, as a general
guideline, approximately 1/64" or 1/32" of relief is typical. This
is generally measured at a fret that is at the mid-span of the neck
(typically the 7th fret) and is the distance from the top of the
fret to the bottom of a string when the string is fretted at the
first fret and at a fret where the neck joins the guitar body,
typically the 14th fret. By simultaneously depressing a fully-
tensioned string against the first and 14th fret, the string forms
a straight edge spanning the first and 14th frets. Alternatively,
a metal straight-edge can be laid along the fingerboard (on top of
the frets) and the distance measured from the top of the 7th fret
to the bottom of the straight edge.
There's a lot of other good tips on this page here:
http://www.guitarnotes.com/notes/noteget.cgi?basic_guitar_setupHope that helps!
Dave