Hi Lezz
At this scale 1mm is, as near as makes no difference, equivelant to 1 inch. How wide were the original planks? I would have thought 5-6 inches giving 5-6mm. However, what looks right to me might not to you. When I planked my Brittany fishing trawler, I experimented with some spare ply and planking to get an idea of suitable width and caulking methods. For the latter I used black cartridge paper.
HTH Good luck
Doug