Don't get caught up on the m/s issue.
While it's true that the USA is backwards in insisting in 'Imperial' measurements (c'mon, 1776 was a long time ago!), the markings in the THLR reticle are also in mils.
And as
@HingleMcCringleberry said above, Thomas shows how to estimate based on vegetation movement. Won't work for all situations, but where it is relevant, the fact that it's in metres doesn't really matter - just think of 'X observation equals 2', 'Y equals 3' etc, and go for it. Or just go for light, medium, and strong.