I think I’ve got it now. You’re measuring distance in one plane only, perpendicular to the line shot from, so you need a way to walk down the center line and mark all arrows that are parallel to that line.
I’m going to go back to lasers, because lasers! So what if you had a vertical stick with a rotary laser level mounted? You could walk down the center line and the beam would broadcast perpendicular to the line. Here’s one on amazon for $70. You could also get the laser detector for even more accuracy.