Quote fromI know that a monster can spawn in the square at the centre of 4 torches placed in a 7x7 square.
is the darkest point during the night (obviously) and is dark enough to spawn a monster.
monsters will never spawn on these blocks or any blocks outside this area.
I've actually witnessed first hand a monster spawn in this fashion when my courtyard was lit by torches arranged in 7x7 squares in this fashion. I might be calculating incorrectly, but that black square would be at light level 8.
I'm not sure if this has been answered yet.
But from what i've discovered, if the torch is on a wall, that will add another block to the count, it travels down the wall before travelling along the ground, travelling along the flat surface
which meant by the time the light reached the center of your courtyard, it would have been a level 7 square