I haven’t been in this forum for a while. But I think the 400’ AGL from take off point makes sense as it is your only ‘accurate’ reference that you can see from the drone’s telemetry. Beyond that, you are guesstimating your drone’s current height AGL while flying, unless you are on completely leveled ground. Or have a very good mental map of the ground level around you (1.2 mile radius) and be able to calculate your drone’s actual height in your head while flying.
That doesn't make sense. For example, I live on a fairly steep hillside and if I went by my takeoff point I wouldn't be able to fly at all toward the mountain ... which happens to also be the safest place to fly in terms of minimal chance of encountering either people or other aircraft. In the other direction I would quickly be well above 400 feet AGL and at a altitude and position where border patrol helicopters show up in a heartbeat. Going by takeoff point is a useless guide.
I can easily judge whether I am approaching 400' AGl simply by monitoring the video feed. If you can't, you probably shouldn't be flying a drone.