I started learning to use a manual lathe towards the end of the last year and I have seen conflicting advise for the proper height of lathe tools. Often, I see a forum post with advise along the lines of "at center or slightly above" but I have also seen the opposite of "at center or slightly below". I'm curious as to what the technical basis is for the tool height to be above or below center? Is it a function of the type/orientation of the tool or perhaps the rake angle? Is there an authoritative reference for this information, Eg., machinery's handbook?