What if your range finder stops working because;
1) You utilized a lens cleaning agent on the lens instead of just water.
2) The Batteries Died.
3) The mean time between failure just occurred and the electronics are gone.
4) The internal positioning adjustment of the laser came out of spec.
5) Somehow you accidentally broke it.
6) The air is Hazy and you cannot get a good read.
7) The terrain you are hunting in doesn't contribute to solid readings.
8) It is raining just enough to prohibit readings.
9) It's snowing enough to prohibit readings.
10) You somehow left your LRF back at camp.
Now is when you must have either a Mil-Dot / Milliradian or Minute of Angle Reticle and know how to use it. Your Cartridge Specific BDC reticle that your last Instructor recommended and sold you, is now completely worthless. He really sold you a bag of BS.
In addition, your target is elongated when you are aiming up or down on an angle. How do you range your target now?
For information regarding this and other ranging techniques that work, come out and take my class.
Sniper Tools Design Company - angle cosine indicator