sorry if in the wrong category, but i want to cut two shapes, from two different colour materials and have one fit exactly inside the other - how do i work out how much material my laser is burning away ? so i can make one shape larger to allow for the kerf of the cut ?
It is cut directly from the manual 
and there are many good videos on YT that deal with the subject.
(To find the correct fit, find the size of your laser beam and tweak a bit to find the desired fit for the respective materials. Use scraps or waste material to practice with.)
Kerf offset
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Kerf is used to mean the thickness of the cut itself when using a cutting tool. If you use a table saw, and the blade is 2mm wide, it has a kerf of 2mm (or just slightly more). A laser burns material to cut, and although the cut is very thin, it does have width, and this width needs to be compensated for if you are trying to make parts that fit together, like a tabbed box, or an inlay. Kerf offset works exactly the same as the Offset tool in LightBurn, but it happens “on the fly” as the cut data is generated for your laser, so it does not alter the original design.
Using outward kerf moves the laser beam outward, away from the shape, and using inward kerf moves it inward, into the shape. LightBurn accounts for the fact that holes in a shape will need to be offset in the opposite direction, as shown here - the solid lines are the original shape, and the dashed lines are the result of shifting the kerf out or in:
A kerf measurement tool may help:
You can find other tools using the same idea, but the general idea is to measure several cuts at once, then find the average to reduce the error.
You can apply the entire kerf width to one part or half to the part on each side of the cut. It’s easy to get the setting wrong, so practice on scrap before spinning up the irreplaceable veneers.
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