Acoustic Guitar Construction

What is the actual measurement for placing the bridge ?
And, where is the measurement taken ?
My bridge setter says 25.4 Martin Long Scale and I would assume the measurement Is from the inside if the Nut to the inside of the bridge saddle. Am I correct ? The setter reads 25.4 but when I use the setter and measure the distance it measures 25.5.
Thanks in advance !
Andy

The Martin long scale length is actually 25.34" the number engraved on the back side of the bridge setter ------- The tool is labeled 25.4" which is the nominal scale length used for advertising etc. The Bridge Setter tool places the saddle at the correct compensated distance from the nut.