The bridge is placed so that the saddle of the first string is at a
distance of 22-1/2" (571.5mm) from the front of the nut. I
used a compensated bridge model (Les Paul Junior), but compensation
is not an issue on a lap steel guitar as, in practice, intonation
errors can be corrected by holding the steel bar at a slight angle
when playing.
Measure a distance of 22-1/2" (571.5mm) from the front of the
nut and mark a line S perpendicular to the center
line. Alternatively, you can also measure a distance of
11-1/4" (half the scale length) from the 12th fret. Take your
bridge and measure distance X as shown above. Next, determine the
diameter D of the string posts with a caliper. Subtract half of
this diameter from measurement X to get the required distance from
line S; this distance was 5/32" 4mm on my bridge model. If you
work very accurately when marking and drilling the post holes, you
can use this distance measurement and scribe line P parallel to
line S and the bridge will sit right where it is meant to be. But
because the whole bridge can be set further back with two allen
screws, I recommend that you position the bridge (and draw line P)
marginally closer to line S and then adjust the bridge to its exact
position after mounting. I chose a distance of 1/8" (3mm).
Measure the distance between the two bridge posts and mark this
distance centered to the neck on line P. My bridge posts are 81mm
(3-3/16") apart; another common value is 3-1/4"
(82.55mm).
Measure the diameter of the threaded inserts. They are ribbed and
you need the diameter without ribs (in my case 11mm). Drill two
holes of that diameter, then tap the threaded inserts into place
using a piece of wood between hammer and metal for protection.