Temil-stik and pyrometric-cones are good ways to find the temp accuracy. The cones melt and slump down at a particular temp. A set of three stiks or cones will pretty much zero in the actual temp of an oven.
Say you HT at 1475F most of the time. Get 1465, 1475, 1485 cones. Place one of each indicator ( mark with a stick or a cone) in order about 1" apart on a 3X3" sheet of 1/4" steel and set in the center of the fully pre-heated oven ( oven should have been holding at 1475F for 15 minutes). Shut door and wait ten minutes. Open and observe cones. If the oven is dead on, the first two would have changed color or slumped, and the third would be unaffected. Repeat with new marks/cones and adjust the temp up/down as needed until you get the scenario when the first two have slumped and the third is still erect. That is pretty close to 1475F. Observe the readout temp and you know your error factor. If the PID says the temp is 1465F, you have a +10 degree differential. On some controls it can be adjusted, but on most, you just make a note and know that 1465F is really 1475F...and set the control for 1465. As TCs age, thgey may drift, so checking every so often is a good idea. And when you change a TC, check it.
Also, borrowing another TC and readout from someone and comparing the two readings is good. If you look at many peoples (and lab/industrial setups) you often see two or three readouts. That is redundant measuring to get an average. It is important in industry, but not so needed in knifemaking.
What matters is that the temperature YOUR kiln reads is what you HT for. If it gets a perfect HT at 1465F....use that number. Testing and keeping records is how people come up with posts like, "I austenitize at 1478°F ...I temper at 412°F....etc." They have found that ON THEIR OVEN, that temp is perfect. It may be different on your oven, so try higher and lower and compare your test results.
