LoF rules ssay this: "The troop must be able to see, at least, a part of the volume of its target with the size of the target's head, or a minimum size of approximately 3x3mm (the size of the black squares on the Silhouette Templates)." (emphasis mine) Now, I take that "or" as a clarification on what size that "target's head" is supposed to be, and that you either can see that 3mm by 3mm square on target's silhouette template, or you don't have LoF. Other people in my group, on the other hand, insist that if there's a spread out zone of the same size, that's still enough. Usually that disagreement is not really important... but for prone models, it is. Since a trooper in the Prone state has a silhouette equal to his base, his total height is exactly those 3 mm - and thus if he isn't exactly on the edge of a roof, then anyone looking up at him wouldn't be able to see that square => can't draw LoF to him. I.e. prone trooper that's away from edges can only be shot by someone whose silhouette rises at least 3mm higher than that roof; and a prone trooper on the edge that got shot from below and fails a Guts roll has to move away from that edge (and gain total cover). Is that correct?