If you have a prepared bridge that you intend to use (with a saddle pre-cut 1/8" higher than desired approximate height) then skip the next step.
Affix a spacer to your bridge that is your approximate desired final saddle height PLUS 1/8". Typically, I like for my bridge to be around 3/8” tall, the saddle another 1/8” beyond
that for a total bridge height of 1/2”. The overall weight of the bridge is a factor too, but this my typical height.
On your closed guitar body, position the bridge in the proper location and temporarily affix it to your soundboard with binding or masking tape...this position varies of course to the scale
of the instrument you’re building, but will be the same distance from the 12th fret to the bridge as from the 12th fret to the nut.
Loosen the side-clamping wingnuts on the lower edge of the neck carriage, so that the whole block assembly pivots as freely as possible.
Place your thicknessed fretboard (or piece of trim stock the same thickness) in between the body and the aluminum extension, where the fretboard will overhang on to the soundboard as shown.
With the neck carriage loose and tilting, hold the guitar body flush to the top of the jig with the sounboard facing AWAY from you, letting the aluminum extension come in contact with the bridge.
gently pressing forward until the fretboard butts up against the aluminum extension
as well. When fretboard touches and the saddle touches, tighten the side-clamping wingnuts.