Oh, checking for overlap of the same kind of object like you did doesn't work properly. You know about the old 2 families trick? Add 2 families to an object, then check for overlap between the two. Still I wouldn't advice you to use this overlap method here, although it might suit after all.
You could still use the method I described. Some sort of playfield loop was my first idea, to check if there is a block at each possible field position and if there isn't, it will add 1 to the mentioned PV again. Maybe I can come up with something more intuitive when I think a moment about it.