My first thought was to set it so the two beams were aligned at 40 yards and mark the rotating laser. Now when the two beams are on top of each other I know that is 40 yards from me.
This laser setup makes 3 light combinations you can see. The first is where you see the blue dot on the left and the green dot on the right side. This means the target is closer than 40 yards. The next case is where the two dots are aligned and your target is right on at 40 yards. The third case is where the green dot shows up left of the blue dot and this means your target is greater than 40 yards away.
You can scribe 10 yard increments to calibrate this type of a setup. After you have it calibrated then you can rotate the lasers to 10,20,30,40, and 50 yards. The two beams at 50 yards do get to be a little difficult to see in bright sunlight.
I was able to consistently range within about 3 yards of actual measured distance or so out to 50 yards with this setup.
This would really be most useful by setting it at the maximum range you plan to shoot. Then you have a simple in range or out of range measuring device. Plus, it makes it much easier to build as you don't have to worry about making a rotating post.
Not too shabby for something that cost me less than $20.
