stick to one's last

Keep to what you know and don't interfere out of your province, as in Let me handle the defense in this suit; you stick to your last and track down more eyewitnesses. This adage comes from an ancient story about a shoemaker criticizing a work by a Greek painter named Apelles, saying that the shoe in the picture was not correctly portrayed. After the painter corrected it, the shoemaker pointed out an error in the leg, whereupon the painter said, “Shoemaker, do not go above your last.” Over the centuries the story was repeated, and the expression still is sometimes put as cobbler, stick to your last, even though cobblers are nearly obsolete.