...Is it possible to calculate distortion from the original Pic posted? ...

Sure, its simple, its just a matter of Voltage ratios.

Since each horizontal line in the pic is 10db, you can easily see that the second set of side bands are shown to be about 30db down from the first set, and the third set of side bands are 12db further yet, thus my 3% distortion approximation. If you want to nit pic, its more like 4%, but still not all that bad for Comm audio.

You would do well to memorize the following sets of SIMPLE Voltage ratios, as they are always used;

The first set of Voltage ratios are:

3db is a ratio of 0.707:1, or 70%

6db is a ratio of 2:1, or 50%

12db is a ratio of 4:1, ir 25%

while memorized as another "set" of values

10db is a ratio of 3.14:1, or approximately 30%

20db is a ratio of 10:1, or 10%

30db is a ratio of 31.4 : 1, or approximately 3%

40db is a ratio of 100:1, or 1%

and on it goes.