When the scratches are even overall, you are getting close. This works much better than a wooden batten. The ruler is still quite flexible, and makes a nice curve easily. Tilting the ruler will regulate stiffness. The thinner edge is easier to sight under than a wooden batten.
Wood is nice for longer pieces.
Use your legs while sanding and planing. The motions are smoother than simply using your arms. Keep a face mask on while sanding. You don't need to deeply inhale the sawdust. I had to sharpen every 1.5 hrs on average.