What Bugs said. 100' is a bit much for the cheap laser levels but the
principle is the same.

Bugs wrote:
Laser levels, like all levels, need to be tested and adjusted before
depending on them for a level line. The so-called peg test only takes a
few minutes and assures you that the level is truly in adjustment.
Use two points about 100 ft. apart for temporary bench marks. Set the
level up close to one of the benchmarks. Take a rod reading on both
benches and note the difference in elevation. Then move the level to a
point near the other benchmark. Take another set of readings. The
difference between the readings is approximately twice the error of the
level.
Adjust the level until both sets of readings are exactly the same. Then
you have a level outlook on the world.
Bugs