Here's a website that shows proper chair height for sewing and other tasks:
http://www.osha.gov/SLTC/etools/sewi...iondesign.html
My machine is set into a cabinet. I have found that raising my chair as far as possible to allow my legs to fit under the machine results in my forearms being even with the table surface. That is basically what the OSHA site is recommending. It doesn't allow me to sew for 6 or 8 hours daily, but it does extend the amount of time I can spend sewing without waking up stiff and sore the next day. I could not quilt sitting down for more than 2 hours a day or so; standing up I can quilt twice that and still quilt the next day.