Processing image of a laser pointer on a ruler

Hi Daniel! I am a hobbyist newbie and wondered if what I'm trying to do is rocket science or has already been figured out. I am not an image processing guru, so please be understanding if my question seems basic or absurd.

If I am projecting a laser line or dot on a measurement ruler located 8-9 feet away, is it easy to train the image processor to identify the exact measurement of the laser line on that ruler and return the value e.g. the laser line is at 128mm on the ruler (which I can see with my eyes)? For example recognize the value where the laser is aiming on the picture attached. I would like to develop an app for iphone or arduino that could do that. Any help is very appreciated. Alternatively is it easier or more precise to use the ruler as an reference object an derive the distance between the edge of the ruler and the laser mark?