I’ve saved the StartupDocument.ZBR file in the startup folder with my new height/width settings and with a gradient range of 0. but when i restart zbrush i see my saved document for a short moment with no gradient but then zbrush does something and this stupid gradient appears again 
seems like zbrush is loading any other scripts or settings or whatever.
I get around this by creating a macro to get rid of the sphere at startup and make the background gradient default to a gray background