Pre-painting I reduced the thickness of my board to 19.3mm. by putting it through a friend’s sander/thicknesser which is a highly accurate machine and ensures flatness. I did not sheath with glass, but after epoxy and paint the thickness was still under 20mm. I measure the thickness with a set of digital calipers with good length arms. I would guess that you should allow 1.0 – 1.5mm. for the full process.