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.
I have purchased foam yoga blocks that measure about 9″ x 8″ x 6″ and put them under my elbow when I am doing repetitive sweeps during polishing. Maybe it doesn’t seem like much, but when I’m in the groove, I will sit at my machine for several hours at a time. Have a good adjustable chair so you are at a good level and try not to hunch over while you work. Your back will appreciate it!