They Like It
Notch Width Test Pattern for 1/8 by syvwlch is licensed under the Public Domain license.
© 2008 by syvwlch
Instructions
I'm not sure how engraving works, so you may need to adjust the text labels in the design to fit your laser's requirements. Other than that, here's how I would use this, but I'm sure there are other possibilities:
1. Cut this pattern in the same material you plan to use. If you have the room, I would suggest sticking it in the corner of a prototype design. It won't help with that cut, but it could help the next one.
2. Once the test pattern is cut, try sliding a scrap piece of the material in the notches until you find the one with the snuggest fit.
3. Read the value engraved next to the chosen notch. This is not the real width of the cut but the width that notch was in Inkscape or your software package of choice.
4. Adjust your design to use notches of that chosen width in same software package. Easiest way with an existing design with 1/8" notches would be to scale the entire design appropriately, e.g. if you chose 0.121" your design needs to be scaled to 0.121/0.125 or 0.968.
If you disagree with the math, or have another comment/suggestion, comments are welcome. This is Public Domain, so you can also modify it to your heart's content.