What's come to my attention though is that ruins is another matter altogether. Wherever the barrage hole is exactly, it would hit the highest floor (e.g. if there is a 3 story ruin with a ground floor and the centre of the barrage has scattered to the first floor. Then that blast counts as hitting that floor, and only models on the same plane can be affected).
Then you would use true line of sight from the centre to determine if the model is partially behind cover or not, just like you would anywhere.
Although if you are in area terrain (e.g. the model is on the base of the ruin, or in a crater), you will still always get your area terrain cover regardless of where the blast may land