The best way to determine what value to enter for the Parameter field (dB down from resonance) is to choose where the curve is steepest around resonance. This will maximize signal to noise ratio for the calculation.
For example, if the speaker has a high Q (sharp peak like many micro speakers), then -3dB is usually a good choice. If the speaker has a low Q like most tweeters, there may not even by a -3dB point below resonance. In this case, a smaller value like -1dB should be chosen.