If your talking about the three adjustable hard tail bridges sold by c. b. gitty, adjust the bottom string all the way out, top string all the way in and middle string in the middle between the two other strings, mine was 5/16" from the front of the bridge to 25" scale mark.
I am wanting to understand how to know where to drill the holes to locate the bridge
Measure 25 inches from the nut to the body. Put some blue tape on the box and draw a line at the 25 inch mark. Set the bridge on the box so that the 25 inch line is even with the spot where the strings would rest on the saddle.
Use a nail and tap small holes where the 4 mounting holes should go. If it is a neck through design and you want the bridge to be secure, glue some scrap pieces of wood to the neck at the area where the bridge would sit, so that when you drill the holes to mount the bridge, the holes are drilled into the neck wood, not just the top of the box.