Ah, I see the error in communication. Neither the Blast Focus nor the Prone marker can be at 1+1/inf, 1+1/inf: that position is in free space on nothing. They both only occupy 1+1/inf, 1. This puts them on the roof, on the edge. Everything between 0,0 and 1,1 is occupied by the building. All that's needed is for you to draw an line from the Blast Focus downwards. You can do this from the position immediately superjacent to the edge of the roof. And I agree it's unnecessary to chase such precision for the prone marker. I'm just using it to point out that 1+1/inf, 1 is 'on' a horizontal surface if the horizontal surface extents from 1,0 to 1,1.