There is a gap in the rear of the target circle. Inside that gap is "rear". Not in that gap is "flank". If there is no gap in the target circle then all positionals will automatically land and it doesn't matter. If you're forced to the front of the target, use True North.
You have about 7 seconds and a giant blue circle indicator on the ground when Verblinding Scourge is about to go out, which is plenty of warning to prepare to activate True North. But really it begs the question, if there is someone in the party who needs to be able to see the target circle for positionals, why is that person not using the LB instead of a caster?