Here's my home-made 25.5" scale length gauge. It's made from a length of aluminum door threshold strip. After marking it off carefully with an Exacto blade using an existing 25.5" scale length guitar, I filed V-slots along the edge, so that when I mark off my CBG necks, I can press the knife point right into this groove for a precise marking. Also, the slanted edge allows me to get the V-slots right down on the neck for more precise marking. When I mark off the fret slots, I just push the top edge against the installed nut and mark on down the neck. Position markers and bridge placement are helpful additions to the gauge.
I printed out a scale on w fret then taped it on a piece of 1/4 " stock
. Bob's u r uncle.