Interesting.
From my reading the 15mm moves are indeed best worked out by halving the distance in paces for mm and dividing by ten for cm:
200 p becomes 100 mm = 10 cm
320 p becomes 160 mm = 16 cm
etc etc
What struck me was the practical need for measuring sticks divided into 20 mm sections.
I was also struck how this didn't quite mesh with this from page 3 of the rules "Distances specified ... are multiples of .. element widths (... 40 mm for 15 mm ...) each representing 80 paces. There are loads of moves, in paces, that don't divide by 80.