The NTK sensor comes with a calibration resistor built into the connector. As long as the resistor stays in place, calibration is relatively accurate on the diy stuff. Additional calibration adjustments can be made by doing a free air calibration or apply a known concentration of gas to the sensor. The additional calibrations may require a pot to fine tune.
I've heard you can heat old O2 sensors with a torch to burn off contaminants and extend their life. I haven't tried it, but there's nothing to lose by seeing if it helps.
Jim