Do the same for the barrel light.

Shine the barrel light through the muzzle onto the white surface behind the rifle.

The barrel light should have a bulb not larger than muzzle diameter. A small single LED flashlight is perfect for this.

Lights that can sit in place are very handy because it's not possible to hold two lights in two places and tighten rings at the same time.

The brighter the lights, the further from the wall you can have the rifle, and the more accurate this method will be.