Ok, I deided to test it out on some orcs in Sro. I pulled a bunch to an orc camp and parked them with root. Then blasted them.
Now I'm assuming the cone starts at the player and not the mob you cast it on. When I did some later tests I realized the range was shorter. I was standing farther back at that time from my target. In these screenshots, I'm standing a couple yards behind the target and I hit F9 once to get an overhead view. In this view I can't circle around my avatar so what you're seeing in the centre of the screen is exactly which way I'm facing at the time of the cast.
Here's the shot of them right before I casted.
And now as you can see the orc on the right is left standing undisturbed, confirming that indeed the cone edged far more to the left.
To illutstrate it more I added some lines on the first screenshot to show the approximate range of the cone.
