How can I calibrate the axis steps/mm using a smoothieware board controller?
The same as you would for any other board.
From laser-diode-wiki.com
Calibration of the axes
If, despite everything, inaccuracies still occur, the axes can be calibrated even more precisely with a LightBurn function. To achieve this, draw a rectangle (I would recommend at least 100 × 100 mm) and have it lasered. Now you can measure the actual distances and enter them in the following LightBurn wizard (Edit ⇾ Machine settings). As an example, I measured 102 mm on the x-axis. The wizard outputs the new value for the steps/mm of the x-axis ($100) and can also save it directly in the firmware. Thereafter, you let the rectangle be lasered again and check if the measurements are exactly right.
Lightburn as a feature precisely for that:
@MikeyH, I don’t know if happens only for me but this is what i get when I hit the wiki link you left:
Don’t know what happened, but this one is what it should have looked like. Melvin’s site is for Sculpfun, but a lot of the information applies to all diode lasers.