Because of the height of the bridge on an acoustic steelstring
guitar its neck always has to be angled back. Unlike on electric
guitars there usually is no possibility to adjust the bridge height
(you only can file the saddle lower, shim or replace it). Therefore
everything has to be fixed and adjusted during construction. This
needs practice and experience but the authors William R.
Cumpiano and Jonathan D. Natelson have written down some
helpful measurements in their book Guitarmaking: Tradition And
Tape the bridge to its position and put a long ruler on top of the attached neck. The airspace between ruler and top of the bridge (without saddle) should be in the range of 1/64" to 1/16" (0.4mm to 1.6mm). After stringing up the guitar this will result in a medium action.
It may be interesting how great the required neck angle on an acoustic steelstring guitar is. Some degrees or less? You can determine this by a drawing too, but this is less accurate. At first I want to indroduce the "ideal" bridge:
The "ideal" bridge is 3/8" high and the saddle prodrutes 1/8" so the overall bridge hight is 1/2". Two thirds of the saddle should be in the saddle slot.
If the neck is attached at an angle of 0 degree (90 degrees) to the body, the resulting string action turns out way too high.
For an ideal situation the underside of a ruler on top of the frets should go over the bridge at a distance between 1/64" to 1/16" (0.4 to 1.6 mm). More airspace for a more flexible neck or top or both and less airspace for a "stiffer" guitar. The airspace must be within this range (1/64" to 1/16"). This is the way how you should determine the neckangle in practise. The following calculation is only theoretical.
The neckangle is calculated by:
tan alpha = h / L alpha = arc tan ( h / L ) h = ( bridge height + airspace ) - ( fingerboard height + fret height) L = ( scale length + compensation ) - distance from nut to 14th fret
On a "stiff" steelstring guitar with a 25.5"
scale length the neckangle is about 0.6 degrees
On a "flexible" steelstring guitar with a 25.5" scale length the neckangle is 0.8º
You see it's below 1º and very small but neccessary.
If you like you can now calculate the little airspace "x" between top and underside of fingerboard at 21.fret.
x = tan alpha * ( 21st fret - 14th fret )
It's between 0.95 to 1.34 mm ! This explains the "drop off" of most fingerboards past the 14th fret because the fretboard is pressed down when it is glued to the top. By making the upper face brace slightly convex you can raise the top near the soundhole enough to achieve the neccessary ramp for the fretboard.
Because there should be a sufficient amount of force to drive the guitar top. If the string angle over the saddle gets to shallow (left drawing), the guitar becomes less sensitive, more silent, less responsive (Red arrow = Force that drives guitar top).