Im sure this is old news to some of you. Its all new to me. I used to lay out my thickness lines with a compass. It worked good. The other day while tryng to lay out the nastiest stave Ive ever touched I tripped over an idea in my head. I dont have three hands so you will have to use your imagination, its quite simple. The stick of wood is a 1.5" x 4" chunk of something off my shop floor. The marks arent any certain dimension, just gauge lines I use. I cut the groove in with a rasp edge and rat tail. My forefinger knuckle keeps my intended line right on the bows back/side line while my right hand gently keeps a pencil pushed down and into the groove as I pull from handle to tip. Boy does it work good, fast and super easy. The best part is that it follows every nook and cranny the back has to offer, thats valuable to fine tiller. I always lay them out parallel. The taper comes in later for my style of building.