You can't use the HeadRoom graphs to EQ successfully with any precision. The graphs are made on dummy head w/dummy ear and canal. The response you see is indicative of this total response. Due to the wavelengths involved, and their interactions with the ear and headphone pads, etc.; you can not use any of the measured data to apply specific corrections over about 2kHz. However, under 2kHz, should be pretty accurate. I should note: something looks wrong with this measurement. I am not aware of the HD280 having such a steep, dropping off bass response.
Now, you can make specific types of measurements to correct response over 2kHz; but this is a combination of measurements and must use a special (different) measurement jig to obtain actual driver/enclosure response; similar to making anechoic measurements of a speaker without room effect(s).
The only EQ you can safely do without special measurement/analysis is broad tonal changes, such as overall treble, or mid or bass.
-Chris