I've always just eyeballed it when I've made terrain. Games Workshop says that 40k is "28mm scale" but it's a dirty lie... it's actually its own scale... the average model's eye level is 30-31mm, but they don't scale with "heroic" 30mm ranges like Reaper's Dark Heaven, for example.
The roads for Flames of War that Galeforce 9 sells (part of their 'Battlefield in a Box' range) actually works pretty well for 40k- we've got a set of it at the FLGS, and even though it's meant for a MUCH smaller scale, it seems to "scale up" just fine.