I agree with Debbie C, I would think it would relate to the length of your forearms. Try sitting at the table with your elbow about 3" from the edge (not on the table). Make a fist and measure from your knuckles to the edge of the table. That would be where I'd want the machine needle to be. For me it's about 10½".
My acrylic extension table has 7" from needle to edge, but I set the whole thing back several inches to compensate when I have to use it.