Calculate total harmonic distortion
The amplitude distortion plot you showed is in dB, dBV specifically (dB's relative to 1 V).  So you need to take those dB values and covert them into volts before putting them into your formula.

For example, -67.363 dBV is 0.000428 V (RMS) (using an on-line calculator).

You should be able to take it from here.