Shelving Equalization for Subwoofer

Although I understand the result of a shelving eq to flatten out response, and I know I'd like to have an active eq for this job, I don't really know how to figure out what slope I need to use to get the desired response. I know the subwoofer can handle the extra excursion of an equalized output.
The driver is an Adire Audio Tempest in a ~4.5 cu ft enclosure.
I calculated it's -12dB point to be 17.87Hz and it's -3dB point to be 35.02Hz.

What I'd like to do is design an eq that results in flat output disregarding room gain, then add a pot into the mix so that I can adjust the equalization to account for room gain. That way, I can get reasonably flat response nomatter where it is by adjusting the magnitude of equalization.

I can build a simple parametric eq, but I have no idea what frequency it should centre on or what Q (or Q range I suppose) I should go for.