One way is to use a fret locater chart and a metric ruler to mark the fret positions, and then to measure the distance from the nut to the twelfth fret. Use this distance to measure from the twelfth fret to the position where the bridge is to be located.
Another is to copy the measurements from another guitar, and mark them on your new build.
Or, you can make a special gauge from some material (wood, plastic, metal, etc.) which has little notches cut out on the edge to indicate where the frets and bridge should be. This is what I've been using, and it has worked great. But, I wanted something different and a little more sexy than a stick with notches.
If you don't mind spending $38 plus shipping, you can purchase a really cool little tool from StewMac, which will do the bridge location job very simply. But, when I looked at what I would be getting for 38 bucks, I decided I could make the same thing for about a buck-thirty-eight.
A couple pieces of discarded oak and maple, a brass rod left over from when I was using it for nuts and saddles on my earlier creations (cut long enough for it to be used on short-scale builds or on long-scale bass), and a nut and screw from the many bins in my shop, a drop or three of super glue, and voila . . . I got a real honest-to-gawd adjustable bridge positioning tool, which even looks better than the 38-buck deal, and it works perfectly.
Here it is, a simple, adjustable bridge locater.
The half-inch oak piece, shaped just like the maple, is drilled slightly
larger than the rod, so that it will slide easily on the rod. A hole is drilled
through the top to join the other hole, and a nut is countersunk and
glued in place to accept the turn screw, which will lock the slider in
A shallow slot is cut into the base of the oak slider, so it will rest on
the twelfth fret.
The finished tool is ready to use.
Place the tool on the fretboard (after the frets are in place) with
the tool resting snugly against the nut. Move the slider to a position
where the notch rests on the twelfth fret, and lock it in place with the
Lift the tool off the fretboard, turn in around 180 degrees, rest it once
again on the twelfth fret, and mark the bridge position, which is defined
by the end of the tool.
It's that simple, and effective . . . and only a buck-thirty-eight!