Well…the most important reference in a mix and then mastering context should be your ears tied to a known acoustic transducers system (speakers and the room they live within).

Coming to previewlounge chart (I’ll be quite obvious in what i’ll write)… you can see (left to right) the audio spectrum and going down to up you can see some typical foundation instruments and where that audio spectrum characterizes their typical “qualities”.
So it’ll be useful in a mix situation in cases you lack some of those qualities and want to get back while balancing in respect of other instruments sharing same spectrum’s area.
In mastering context I think this chart would be “less useful” for surgical EQ duties.

Another nice (interactive) chart is at the following link:
http://www.independentrecording.net/irn/resources/freqchart/main_display.htm

It’s reported also the so called Fletcher-Munson chart about Ear Sensitivity

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