Ideally you'd use a pair of matched gage blocks on a granite surface plate and set the depth micrometer base across these blocks. You will then be able to verify the micrometer against the blocks. Do this for the beginning and end of each rod's range. In other words, check the 1-2" rod at 1" and at 2". Less reliably, you can use just one block for calibration. In this case make every effort to keep the depth micrometer rods perpendicular to the granite surface. Gage blocks are available.