Hi Jeff,
There's not so much a minimum height as a minimum amount for it to see. If you look at our seeker platform, the tracking camera is only 3cm above the ground, but to compensate for this the drone sits leaning back so that the camera is angled slightly upward when it's landed. If your drone sits on the body and the camera is facing down, you won't be able to take off with VIO, so you'll need to find some combination of height off the ground and angle that allows the camera to see a decent amount of its field of view in order to take off successfully.