Has there ever been a bridge on it before or is this a new built, virgin?
The final mounting position is established after the neck is installed so you can measure the correct placement of the bridge from the neck's 12th fret or octave point.
In a new installation (the virgin) I'd leave the barrels at the the 50% position but just asva guesstimation to get the bridge in a decent enough position to later have enough travel for intonation.
Other than on a virgin that is, you're going to need to adjust the barrels anyway unless you have some strange plan to not need to set it up after you've got the bridge installed.
Come to think of it, I you DO have a magic formula to presetting the barrels, that would be nice to tell how to do it for all the Les Paul Special Epi/Gibson crowd. Gawd --- I hated adjusting them and then loosening the strings to access and lock the setscrews when it was adjusted.
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