For someone else googling how to set heightmap so it’s at 0 elevation, the closest RGB value is #959595. (at least for 16bit color space, should be the same for 8bit).
I would really appreciate if this was in the documentation, the value itself doesn’t make sense to me, had to find it by trial and error.
Just create a flat landscape and export the heightmap. You’ll see the default value that way, which should be 0.5 * max_value. (a middle grey)