I recently purchased a 1996 XJ6 which was not running due to various motor issues. I wish to document the means I used to set the TPS to the correct 0% throttle position.
After ensuring all sensors are functioning properly, auger the mounting holes of the TPS to allow physical rotation. The TPS is merely a variable potentiometer, what we want initially is to create an out of range low setting.
With the TPS set to full low, i.e., the voltage at pin 2 is approximately ground, the ECU will induce an out-of-range condition and assume a throttle position, which in my case was 24% indicated by my OBD2 reader. With the engine running, slowly rotate the trottle plate and accelerate the engine. The TPS will sweep past the Zero value causing the engine to hesitate and drop in RPM. In this condition the TPS is reporting a closed throttle plate, but the O2 sensors and MAF sensor indicate otherwise; as a result, the engine oscillates between an idle and accelerate condition. One can note the position by back probing the voltage on pin 2 of the TPS using a multi meter. With the ignition on and engine off, loosen the torx screws securing the TPS to the throttle body and rotate the TPS until the voltage on pin2 is the same as that corresponding to the closed throttle position. Tighten the torx screws and start the engine. If all is well, the idle will settle to the expected values.

Alternatively, rather than probing the pin 2 voltage from the TPS, one can use the throttle position value from the OBD2 reader to observe when the the TPS jumps from the out-of-range condition (24%) to the 0% throttle. I found that it was important to invoke the out-of-range condition first before finding the 0% throttle position, I am not sure why I could not just use the TPS value directly to set to 0% open.

Final note: While this may work for setting the closed throttle position and idle condition for the engine, there could be a deviation from calibration in the sensor reading along the throttle travel. In some circumstances the error may be automatically corrected with the MAF and O2 sensor readings, but for perfect operation it is recommended to go to the Jaguar dealer, offer your firstborn, and have the throttle position calibrated properly.