Distance is counted without regards to structures. Meaning you can just count 2 letters higher/lower and 2 numbers higher/lower making a nice simple square grid. Same applies to 4 spaces away.
This means you can through over structures and the affect of almost all equipment also travels thru structures. The flash back is a bit of an exception since the blinding light it generates does not go through structures.
Note: the equipment must "land" on a spot with coordinates.