Normal map rule of thumb?

Should high poly mesh to bake normal be “over” the low poly map? Or can it go back and forth along the low poly mesh. I am getting all kinds of results. Some times relief detail on high poly gets wiped out. I know the “Distance” and “Bias” setting on Bake tab has lot to do with it. They both take positive numbers only. Does that mean high poly mesh must be “over” the low poly? I can tell what the Distance settings do but I have no idea about Bias. Any hints?

did you check out the last video last week on head retopology from CG cookie

in 2.5 i mean seems that normal map are a lot better in 2.5 them in 2.49

and even if you rotate object you can still see the bump quit well
which was not the case in 2.49


Experimented some more. High poly model do seems to need to fit over the low poly; Bias setting has not negative number. Also it is bad idea to put normal feature too close to the edge of geometry. It just doesn’t look right. Here I put camphor right at the corner of tube but it looks flat. The indentation in the middle came out fine.