two ways I can think of
1.get a slotted 12 string nut.
2. from the bass side of the neck
All in mm........4,7......11.5,14.8.......19.4,22.........27,29.8.........34.5,37.5....
.....42.5,45
I just got these off my plans, it looks as if the second string of each pair is roughly 3mm from the first. I put in fractions of mm to show that they all aren't exactly 3mm from the main strings.
If it's any help the centres between the holes on the bridge is 11mm that's on both rows and then the centres between the string pairs is 4mm
Draw it full scale and see what it looks like