The neck must be fastened to the body before you are going to position any bridge. An important factor that determines where the bridge will be situated on the body is the point where the neck meets the body. On two guitars with similar body shape but one with a neck to body connection at the 12th fret and one with the neck to body connection at the 14th fret, the bridge is closer to the lower body end on the first guitar.
An acoustic guitar usually doesn't have an adjustable bridge. The necessary compensation is generally build into the bridge and achieved by a slanted saddle. Acoustic guitars with six individual positioned saddles have a disadvantage: They work fine only as long as the strings are new and as long as you stay with the same string gauge.
In practice a fixed compensation like the one shown above works very fine for a wide range of scale lengths. A compensation of x = 0,15" (3,81 mm) works for shure for scale lengths of 25" (635 mm) to 25.75" (654 mm) but it might work for a shorter scale length of 24,8" (630 mm) too.
Generally you need more compensation with a shorter string
length so x = 4 mm might be better for the 24,8" (630 mm)
scale lenght. (A mandolin requires 6 to 6,4 mm!). If you are
uncertain on a scale length below the "safe range" you
can use a thicker saddle. Now it's possible to change the point
where the string rests on by a small amount if necessary.
The middle of the saddle is positioned "scale length plus x" off the nut. The front of the bridge must be exact parallel to the frets and the whole bridge must be centered to the fingerboard. The string action must be "normal", the given compensation doesn't work if it's to high.
Thanks to William Cumpiano for this information. Visit his web-site at www.cumpiano.com, subscribe to his newsletter and get first hand information.