Thank you so much for your kind words :) and I love that you asked! The thing about the ruleset is that it is meant to be used on a table with twelve-inch rulers, which is why characters’ speeds tend to be from 12” to 3” with four degrees total.
The issue is that metric rulers go up to 15 cm, which is not divisible into four degrees of speed. However, at the same time, I think it is extremely difficult if not impossible to get to a speed of 3”. The only way I can see someone exceeding 1,500 coins of weight is by wearing plate armor and a backpack, wielding a two-handed sword, and carrying a large sack – which is pretty silly. If only speeds 12”/9”/6” are realistic, then we have a basis for saying metric speeds might be 15/10/05 cm. (Though still, these are not proportional to each other, since 6” is half of 12” but 5 cm is 1/3 of 15 cm.)
Overall, the goal of FMC is to be an accurate representation rather than a reformulation. I would really like to make it more accessible to people who use SI units, but base-12 math is just not friendly with base-10 math. At the same time, if you wanted to use SI units, it shouldn’t be difficult along the lines of above! It just would not be the “same”.