The Bob Nelson CW voltage deciphering chart for use with Harbor Freight & similar meters:

 

The RMS voltage should be 18 volts; but your meter should read a little more than 15 volts.  Here is a chart for converting meter readings to RMS voltage:

 

Meter          RMS 
  0          0
  0.5          1.6
  1          2.7
  1.5          3.7
  2          4.6
  2.5          5.4
  3          6.1
  3.5          6.9
  4          7.5
  4.5          8.2
  5          8.8
  5.5          9.5
  6          10
  6.5          10.6
  7          11.2
  7.5          11.7
  8          12.2
  8.5          12.7
  9          13.2
  9.5          13.7
 10          14.1
 10.5          14.6
 11          15
 11.5          15.4
 12          15.8
 12.5          16.2
 13          16.6
 13.5          16.9
 14          17.3
 14.5          17.6
 15          17.9
 15.5          18.2
 16          18.5
 16.5          18.8
 17          19
 17.5          19.3
 18          19.5
 18.5          19.7
 19          19.8
 19.5          19.9
 20          20

 

One problem is that your meter is designed to measure sine-wave AC voltage, which is not what the CW80 puts out.  Nevertheless, a reading of 12 volts should mean that your actual RMS voltage is around 16 volts; so something else may be wrong.  Are you giving the CW80 the full 120 volts that it probably expects?  American power-line ("mains") voltage is often archaicly described as 110 volts, but hasn't been that low for over half a century.  Your meter might also be a little off, which could explain some of the missing 2 volts.