As title says can you start a cone at a back corner?
x= effect, -=blank, o=caster
--x---
--xx--
-oxxx-
------
The example in the book is like this.
--x---
--xx--
--xxx-
-o----
I have always allowed it but it came up in a game and just wanted to know the RAW of it.
Correct, you choose which intersection/direction you want it to go.