I too used one of those things, back in the late Ploesticene era, when there were no other portable rangefinder alternatives.
Jodum's suggestion on how to calibrate it is a good idea.
As to how well it worked for me, it was maybe better than the naked eye. . . maybe. . . once in a while. . . . if everything lined up right, and said your prayers and been good that day. Most of the time, not. The cheapest, worst laser rangefinder is 10,000% better.
Unfortunately, those rangefinders are too light to make good canoe anchors.