It's completely relevant. It clearly shows front, flank, and rear on a target.
The size of the target is irrelevant. "Rear" is the gap in the target circle. You just need to half step back and forth over the spot where the circle ends. Here are some more explicit visual aids:
You can be farther inside, as long as you're inside/outside of the cone drawn by the green lines, but there shouldn't be any reason to be so far inside the boss that you can't tell if you're hitting flank and rear or not. Even super tight donut attacks still let you stand on the circle, and if you're in a party doing savage, they should all know where the flank/rear boundary is and stack to accomodate if necessary.