Here's a simple way to calibrate a pyrometer:
1) Place a self-supporting witness cone in a central area of your firing chamber where you can see the cone through a peephole.
2) Fire the kiln. During the last hour of firing, the rate of temperature rise should be 108°F (60°C).
3) When the witness cone bends so that the tip of the cone is even with the top of the self-supporting base, adjust the pyrometer to match the Orton cone chart. (Use the 108°F column in the chart.)
Note: The self-supporting cone is very slightly over-fired when the tip touches the shelf. The tip should be even with the top of the cone's tapered base.