As it stands at the moment we have been using 5th edition mentality with treating the whole piece as area terrain, and 4+ cover for being a ruin.
The rulebook states the following:
All ruins are difficult terrain and provide a 4+ cover save.
A ruin might be mounted on a base, decorated with rubble, and
other debris. In this case, treat the base as area terrain.
If the ruin has not been mounted on a base, then the ground
floor is not counted as either difficult terrain or area
Area terrain is always difficult terrain. Models in area
terrain receive a 5+ cover save, regardless of whether or
not they are 25% obscured. Models that Go to Ground in
area terrain receive +2 to their cover save, rather than +1.
I've come to realise that only the base of a ruin is area terrain (5+), while the remainder uses true line of sight to determine if the model gains a 4+ cover from the ruin itself.
In my opinion we should start following this ruling properly from now on.