There are two different things to take into account when calibrating a mic. First is zero, which is rather easy. Second is accuracy. That is not very hard to determine, especially with gauge blocks, but not easy to adjust.
First. Setting zero, especially on a 0-1" mic is as simple as running it closed, and adjusting the barrel with the included, but often misplaced, spanner wrench. With larger mics, a standard is used to make the anvil contact the spindle. Make sure that both the anvil and spindle are perfectly clean, and free of any debris, even lint!
Second. Checking accuracy is easy. Take a gauge block and measure it. If the block and the mic read the same, you are golden. Do this with a couple of different measurements, properly wringing the blocks to ensure accuracy, to verify accuracy along the entire stroke of the mic. If you do find discrepancies, Unfortunately, I do not believe that it is correctable, without replacing parts. This usually occurs due to wear, often at the small end of the range. Maybe someone else knows if they can be adjusted.
-Cody