The actual math is "impossible" since it requires two point measurements of loudness (to, from).
That said, the curves remain very similar for different volume deltas and , in my experimenting, an approximate curve produces great results.
The +/- % knob is exactly right. You'd basically dial in the dB diff between reference (loudness making the patch) and current (loudness playing patch).
The curves are mostly similar, so going "up" in volume relative to the volume which the sound level at which the sound was dialed in, one would need would subtract a fair amount of bass and a little bit of the very high end. The 1k - 6k range part of the curves stay relatively the same shape all the way from 0dB to 120dB, so the main adjustments would be outside that range. The ear is poor at picking up bass until sound is fairly loud, so the rate of change in the correction would be fairly steep on the bass side, as the required bass output level for perceived equal loudness only changes about 20dB between 0db and 80dB. The very high treble has a smaller change in slope above 6k, possibly almost negligible, but if we're in there and adjusting EQ, might as well adjust that too in order to keep things sounding as "flat" as possible with the volume level changes.
If we ignore the range below 40dB, which is pretty quiet, really, we could get a bit finer control over the range of 40-120dB. Things in the 40dB-ish range: "Library, bird calls (44 dB); lowest limit of urban ambient sound" per the site
http://www.industrialnoisecontrol.com/comparative-noise-examples.htm.
The change rates in sub-1k and super-6k ranges are close enough to constant rate on either side of the 80dB curve as to make it a good compromise middle value for the "do nothing" flat curve, and more realistic for a bogey "bedroom" volume starting point. By way of comparison, here's what the previously-referred link has as examples of things in the 80dB-ish range: "Garbage disposal, dishwasher, average factory, freight train (at 15 meters). Car wash at 20 ft (89 dB); propeller plane flyover at 1000 ft (88 dB); diesel truck 40 mph at 50 ft (84 dB); diesel train at 45 mph at 100 ft (83 dB). Food blender (88 dB); milling machine (85 dB); garbage disposal (80 dB)."
And, perhaps reinforcing the choice of 80dB as "middle", typically 85dB is what is recommended for mixing music, as the changes in frequency response as you deviate from that are least detrimental to most mixes, per what I learned when studying studio music recording and audio engineering half a lifetime ago.
Really, though, the whole idea is more at having one knob you can tweak to "un-FUBAR" your tone at a gig or at home (depending on where you dial in the sound) than an exact "this to that" conversion, I think. One could even have the EQ delta curve to correct from 80 -> 120 and the other EQ delta curve to correct from 80 -> 40 stored, and simply blend in either curve with flat, depending on how the knob was turned. Then it's simply a balance pot taper for each direction deviating from "0" that needs to be worked out to give a smooth transition....