According to a witness cone, my controller is 100 degrees too cool, which is beyond the range of the thermocouple offset adjustment.
Your thermocouple may be ready to fail. The reason the thermocouple can be offset only up to 45 degrees F is that an adjustment greater than that indicates a bad thermocouple.
Another possibility: The thermocouple reading will be cooler than the actual kiln temperature if the thermocouple is not pushed far enough into the firing chamber. This is especially likely if the thermocouple is not permanently mounted to the kiln. (Some people use a portable controller on several kilns one at a time, and the thermocouple is mounted through a drilled peephole plug.)
If the thermocouple is properly mounted and you don't want to replace the thermocouple yet, you could use the Ramp-Hold mode and just fire to a different temperature. In some ways, Ramp-Hold is simpler than Cone-Fire, because you can fire to any temperature you want instead of to a pyrometric cone number.
Or you could fire to a different cone number in Cone-Fire mode.
By the way, if you adjust Thermocouple Offset, the controller's room temperature reading will be off. That is normal.