Using this test, if the top number is between 2 and 3 do you round up?
My bottom number is 5.
Following the round up method this would be 3.5 divided by 40 the Kerf would be 0.0875.
this would be the Kerf Offset entered in the cut setting screen, correct?
I am going to be the Sacrificial Lamb here. What are you talking about? Maybe an image will help…
Per the instructions, that’s 2 with 5 as the first decimal place, then divide by 40:
0.063 = 2.5/40
Diode lasers typically have a rectangular spot, so expect a somewhat different kerf along the other axis. Rotate the LightBurn layout by 90°, make another gauge, and compare the results.
Got it. You do not do any rounding. You read it sort of like a slide rule (before calculators).
From Lightburn Docs…
Find the largest number that the line marked D passes on the top scale, and use it as the whole number (D) in our equation (2 in the example).
Find which line on the bottom (Vernier) scale best lines up with any line on the bottom border (3 in our example). This will give us the decimal place (E) in our equation.
In your image, it is reading 2.0. Say you slid it over 2/3 of the way past the 2.0 position. Then it would still be 2.0 plus the reading on the bottom scale, which would be 1 mark past the “5”. The final reading would be in this case 2.6 indicated, but the KERF is 1\2 of that, or 1.3mm.
That said, I have no idea why the 2.6 would be divided by “40” like it shows in the Docs. The file provided in the Docs may be different than the one that was circulating in some postings.
If I Googled the right laser module, they claim typical 0.014mm beam spot. Ideally, your kerf would be .007mm, but wood-burn would likely make it closer to 0.01mm (which is pretty good).
Because each material is different, when I use a downloaded file, like from boxes.py, I will copy a tab and slot to make a test cut for the fit. I also operate with 3 different wattages, so a quick test keeps me out of trouble. I have included a test file that you can use on about any material. I usually get a decent fit by the 3rd try. 
Kerf Cut Test.lbrn2 (28.7 KB)
No, that is where it is used. I am asking why 40 instead of 20 or Pi?
The line of little rectangles has 20 abutting cuts, so it’s short by 20 kerfs: 1 kerf = 20 missing kerfs divided by 20.
The kerf offset is half the kerf width: divide 1 kerf by two.
So the overall divisor is 40 = 2 Ă— 20.
Which had me counting on my fingers, but that’s not unusual …
Wow, that is so fancy! Thanks!
Where is this 20 coming from?
Here is the copy direct from the doc.
Set up the Cut (red) and Score (black) layers with your usual settings, and ensure the Kerf Offset is set to 0 mm (off) in the Cut Settings Editor. Position and run the Vernier Kerf Offset Test on your machine as normal.
Remove all pieces from the machine (a piece of painter’s tape can assist), and remove the piece marked DISCARD.
Slide all loose bits to the right →. The horizontal black line can be used to orient the rectangular pieces if they get scattered. 
Find the largest number that the line marked D passes on the top scale, and use it as the whole number (D) in our equation (2 in the example).
Find which line on the bottom (Vernier) scale best lines up with any line on the bottom border (3 in our example). This will give us the decimal place (E) in our equation. 
Fill in the equation, calculate it, and use the result as your Kerf Offset in the Cut Settings Editor.
D . E 2 . 3
----- = ----- = 0.0575 mm
40 40
Optional: You may then use the box joints included to test your new Kerf Offset.
Once you have found your Kerf Offset, it’s a good idea to save the settings to your Material Library, and the test itself to the Art Library, for future use.
No 20 mentioned?
You want to know how much material a cut removes, this is the kerf.
This makes 20 cuts, to get the average cut (kerf), you divide the total of 20 to get the average kerf. If this made 10 cuts, you would divide by 10, not 20…
Then divide the kerf you get by two. 1/2 of the kerf is on one side the other 1/2 kerf is on the other.
That should get you close to what is actually needed. I always cut a notch/slot that’s only one or two. Then I can see if it will fit like I want.
Basic math… multiplication/division rule it doesn’t matter where it is… 2 x 20 = 40, so using 40, you don’t have to divide by two. I think @ednisley was pretty clear on that…
2 . 3
__
2 x 20
Is the same as
2 . 3
__
40
After all of that, I don’t think you’re getting a proper kerf. The Internet says your laser has a spot size range of 0.011" to 0.014" to → mm is 0.280 to 0.3556 … Kerf must be at least that large and more likely larger than the machines spot size. Depends on the material.
My co2 has a kerf of about ~400 microns/cut.
I’d venture to say you hosed something up. I don’t see how you can possibly have a kerf of 57.5 microns (0.0575mm), that does not seem possible.
Another issue, is that most diode lasers have a square or rectangular spot. If you do anything other than up, down, right or left, such as a diagonal, then the kerf will change.
Make sense?
OK, first I want to thank everyone trying to help in this area of my personal confusion.
on the tool created for my laser, the top number is not on 2 or 3, it is in the middle between the two.
do I use the 2 or the 3 for my calculation or does it not matter?
As you slid the scale to the right, the largest number passed by Line D on your scale was 2, which is why we’ve all been using 2 in our explanations.
It works like a caliper.
Looks like you’re reading it correctly, but something is wrong with the results. If your spot size is 0.20mm, then your kerf will likely be larger.
Can’t say I’ve ever seen one with the same value, or lower, as the tool diameter. They usually take out at more, as there needs to be enough for clearance.
This was the answer I was looking for to start with at the beginning of this thread.
The answers received, although helpful, but confusing at first, lengthened the thread, we finally have a solution.
Thanks to all that contributed.
At the risk of rubbing it in, I pointed to those instructions as a direct reply to your first post …
The way it was worded, I did not get that. Just a simple “2” would have answered the question. I was reading too much into it.
But thanks for taking the time to point out my lack of cognitive abilities. (just kidding) Risk received.
