I think I just realized why #Slant3D uses a 0.3mm first layer, and 0.2mm layers. It means that every 5th layer is at a 0.5mm mark, which makes the math easy for setting the height of the triangular relief cut at the top of holes in vertical walls can come to a point in one layer, and have a continuous layer line above it.

At least, that's my guess.

#3dprinting