>>I have seen the best reliefs done in ZBrush
>>and it was also spoken about on how to do that, with one word. I tried that, it did not gave
>>the correct results. I ask the author, got no reply, asked at #AskZBrush got no reply yet, and
>>asked on CGTalk and got no reply from the pros there.

Yeah, I think there is some difference of opinion as to what is a bas-relief.

Definition of bas-relief from a dictionary
art : sculptural relief in which the projection from the surrounding surface is slight and no part of the modeled form is undercut (see 1undercut 2); also : sculpture executed in bas-relief a limestone bas-relief of a female figure

It is an art in and of itself to create figurative elements that look right. One cannot in general take a full 3D model and scale it down on one axis and expect that to work. I tried that a long time ago ...
I'm not saying that is an invalid method for some things, just that it often does not produce expected or desired results universally.