Ive used a laser, mounted over my bench, it was clamped so i could twist the laser back and forth as one adjustment(for centering the crosshairs on bow center), and the clamp it was attached to i clamped to a joist overhead that allowed for a vertical alignment. I have a long 6 ft aluminum straight edge i would lay across the bench and adjust the laser until it was in line with a square i held to the straight edge. From there the laser was set and i could place the bow on the straight edge and trace all of the corresponding points i needed.
If you dont do this you get lines that trace the curves of your form rather than straight.
String is way better. I use the string