I took the masking/painters tape one step further. I cut a piece about 12 inches long and using a fine Sharpie pen marked the tape every 1/4 inch - the length I wanted the stitches to be. The stitches were going to be very visible so I not only wanted them to be straight but I also wanted them to be even. This worked a treat - just make sure the Sharpie ink is completely dry before you start stitching!