Basically meaning your tank, full to the top with average glass strengths will probably break if you put 50% more force on the glass. The safety factor goes to 2.0 if you only fill it to 50mm from the top.
I only ever fill most my big tanks to 5cm from the top anyway.
3.8 is recommended to account for things like little earthquakes, people bumping into the tank, rocks shifting, fish hitting the glass etc.
Not going to have any big rocks in there just a few big pieces of wood, but they will be partially burried in the sand, and mainly small fish so no smashing into tank.