First attempt to make a pair of interlocking plates on the lasercutter from 6mm ply wasn’t a success. I cut the slots at 6mm, but the cutter isn’t as precise as that and of course burns a little to each side.
cut slots at 5.5mm – pieces wouldn’t even begin to join
cut slots at 5.75mm – pieces would begin to join, but were very tight and wouldn’t go more than halfway without the aid of a hammer
cut slots at 5.8mm – pieces joined nicely, were nice and firm and joined entirely. There is a fraction of a millimetre clearance at the top which suggest I need to increase the slot height by something less than 1/10th of a millimetre in order to get them to join perfectly
width remains the same, slot height increased by 0.08mm. With a wee bit of pushing it seems to be bang on. Reckon that the cutter leaves a slightly rounded corner on each cut due to the nature of the thing so you need to accomodate that.