Kerf is tricky. The kerf for a cut depends on a lot of variables:
1) the type of material 2) the thickness of the material 3) moisture content of the material (e.g. for wood) 4) amount of airflow 5) laser focus 7) laser power 8) laser tube age 9) bed flatness 10) mirror alignment 11) mirror cleanliness 12) cut direction 13) cut speed
Some of these are related to maintenance issues, others are simply facts (or variables) of life.
In general, if it's critical, you should check the kerf on the actual material on the same machine, on the same day, in the same position on the bed...
Once you have an idea of the kerf, you need to shift the cuts by half the kerf.
But I assume you already knew that...
In general, on the laser cutters I use, 0.1mm to 0.15mm seems to be in the ballpark.
In terms of how you do it (apply the kerf to your cutting outline), it depends on the tool you're using (and google is your friend). However, to correctly apply it, the process needs to be applied at a point where the software can differentiate between the job and waste.
In the software I have written, positive polygons are drawn in a clockwise direction, and negative polygons in an anticlockwise direction. The code can use this do determine which way the apply the kerf.
In general, at the point the polygons hit the laser cutter, it's not going to know inside from outside, and so it can't apply the kerf unassisted.