Depending on the brand of your machine, you should inquire from your sw dealer and ask if they have a stitch in the ditch foot. I recently discovered my dealer (Pfaff) has this great foot that has a small metal quide in the middle of the opening of the foot and by keeping that quide in the ditch of the seam it sews a perfect line exactlly where I want it. Ask if ones available. You could be surprised.