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.