At the moment, as far as I can tell step_to and walk_to use get_dist, so a mob using walk_to or step_to stops in an adjacent tile. If tiles are larger, or objects have small bounds, this means there's still a potential gap between Ref and Trg.

I've worked around this by using walk_to until the distance is very low, and then switching to walk_towards, but it's inelegant.