It doesn't really matter where you anchor,as long as you feel comfortable and are consistent with yours.
I shoot split finger ,when my middle finger to the corner of my mouth..
I have taught some that felt comfortable with 3 under.Index finger to the corner of there mouth.
One that couldn't get to full draw but could get a consistent anchor with the thumb pointed up and anchored back of their eyebrow..
So you can see, it doesn't matter as long as it's consistent.