Thinking out of the box here......literally.
I think I have been doing what everybody else generally does and that is to free air calibrate the AFX with the black knob and then next match up hp tuners via the offset voltage formula second.
But......
I am totally reliant on the output wires and never use the AFX box face readings. In reality I only care that the sensor and HPT are in sync. Do I really need to waste a good offset voltage adjustment knob to obtain a perfectly calibrated in between box reading I only look at long enough to say okay before I go spend time "fixing" the real reading I use.
So......
When I am heating up the sensor in free air, why not just use the AFX black knob to calibrate the voltage to read 16 AFR or 5 volts in HP Tuners and move on?