Doesn't matter how much of the target you can see, even if it's only a tiny corner, p60 is clear! If you're in the target's front arc, you roll on front armor. It works the other way around too, if you're in the side arc, you always hit side, even if you can only see a tiny sliver of side, but can see all of the front!
If you can see none of the front, i.e., it's pulled flush up against a building, then you roll on the side, but the target's cover save is increased to 3+, as per p62.