That cutting guide seems odd to me. I more or less cut mine all the way like the photo shows on the left.
I think of the tri-tip like a boomerang , I start cutting at the corner where the two ends come together at a angle, and then just keep slicing parallel to that end. Yet, I always feel that once your looking at it in person, it's pretty obvious which way the grain is really going.
And, ultimately the cook can cut the darn thing anyway they like it, especially if they are the primary eater of the tri-tip too.