Do waypoints use GPS? What is the worst-case GPS error? A quick googling about GPS says "The average best would be 15 meters and the average worse close to 30-40 meters". Based on that info I'd set Litchi at least 40 meters horizontally from an object. And test the minimum altitude locally and add at least 10m to that. And carefully monitor the flight in real-time (maybe with another person at another spot with a cell phone) and be prepared to handle it.