That altitude safety margin must be generous as Hexhaven pointed out. Flew a 6-mile round trip mission with my Mavic Pro1 recently from a takeoff point 200 feet below the destination loiter area. All my previous flights had launched from a takeoff altitude that was higher than most of the destination area waypoints, and this difference in launch point altitude evidently caused Litchi to fly the craft roughly 50 feet lower than intended.
When I played back the video I noticed right away that my usual 150-feet-above-current-waypoint setting had the drone barely skimming the tree-tops, until finally, a tall palm tree trunk loomed causing obstacle avoidance to take over by slamming the brakes and vectoring the Mavic straight back to the launch point in RTH mode.
An easy 90% of my Litchi flights are conducted with my Phantom3 Standard well beyond signal range as is permissible in this forgotten backwater. If I'd flown the Phantom3 instead of the Mavic Pro1 on that flight, the craft would have slammed full tilt into that palm tree trunk, and I would have been none the wiser as to the nature of the drone's fate, given the remoteness of the tropical jungle where the crash would have occurred.
Recently, Litchi screen warnings advising that "elevation data is not available" in the areas I fly, add to the reasons why a generous increase in that altitude error margin is good practice. In my rural locality I would never dream of flying an initial recon Litchi mission below 246-feet AGL now, Just checked my Flight Data odometer reading, and with a total of 114 hours of fully autonomous Litchi missions flown at the standard 17.9 mph cruise speed, the cumulative distance covered is thus 2,040 miles and counting.
Enough rambling from me for now. I've got some more Litchi flight plans to create on the Mission Hub.