Check the contact point area ratio of cast iron ruler

Apply display agent to the working surface of the inspected ruler, grind it onto a flat plate or ruler with an accuracy not lower than its precision, and display obvious contact points on the working surface of the inspected ruler. Then use a transparent thin plate (such as a glass plate) with 200 small squares of 25.mm × 2.5mm engraved within a range of 50mm × 25mm, and place it at any position on the working surface of the inspected ruler. Observe the proportion of the area containing contact points in each square in sequence (in units of 1/10). Calculate the sum of the above ratios and divide by 2 to obtain the ratio of the contact point area of the detected area.