I don’t know if anyone has done a proper ergonomic study on faceting. However, I have some ergonomic background being a retired fire and safety consultant.
Because you will be sitting still for so long, you should have a comfortable chair with a back rest. As far as machine placement is concerned, I have my machine elevates so that when I go to examine a stone or change settings on the machine, it does not require me to bend down or slouch. At that height it also allows me to use the machine or my work bench as an arm rest.