I'm not sure what you're referring to as "three digit calibration." I have two PLC's, and they have two switches and two knobs on the front. One switch controls power on/off. The knobs are for fine speed control at 33 and 45 speeds. The other switch flips between 33 and 45. The PLC is supposed to be used with a strobe disc. VPI makes one (I think mine may have come included with the PLC), but there are many others. Observing the rotating disc in a darkened room under an incandescent bulb will allow you to adjust the fine speed controls at 33 and 45 to correct speed (when the lines stop migrating around the disc). The correct settings are likely to vary between different turntables because of age, wear, lubrication, etc.
VPI recommends that it may be better to change speed control using the turntable's mechanism, i.e., for your HW19II, slip the belt onto the other pulley, rather than to flip the switch on the PLC. I found using the switch was perfectly OK, and I wouldn't have to recalibrate my fine speed control every time.
VPI recommends that it may be better to change speed control using the turntable's mechanism, i.e., for your HW19II, slip the belt onto the other pulley, rather than to flip the switch on the PLC. I found using the switch was perfectly OK, and I wouldn't have to recalibrate my fine speed control every time.