The whole idea behind the 85dB SPL level is indeed based upon the ISO equal loudness curves (technically, the actual Fletcher-Munson curves are rather obsolete these days), and the idea that that's the level here the human ear tends to hear frequencies most evenly (but still not exactly the same).
There is one famous engineer (I forget with one) who is quited in Owsinski's "Mixing Engineer Handbook" as doing most of his mixing at "normal conversation levels", which in a quiet studio CR would probably be in the 40-50dB range, give or take.
The problem is, 85dB is pretty loud. Maybe not rock concert loud or jackhammer loud, but it's loud enough where if you keep things there for extended periods, it can both prematurely fatigue one's ears in the short term and desensitize them to quieter details in the longer term.
85dB is an interesting reference point, it's a good reference to mark on your playback volume control as a point where you can listen with detail without having to crank the volume past that where things can get much more dangerous, but that really should not be warped into the idea of that being where one should do all - or even most - of their monitoring when they mix.
Use what volume is necessary for the specific mixing task at hand at any given moment, with 85dB used as a mileage marker to let you know that's where you're at. When I mix, I have my right hand is on my CR Volume control almost as much as it is on my mouse or my faders; I'm constantly changing playback volume to match whatever it is I am actually doing at the time. Sure I will often crank it up to 85dB to check general playback, or when I get up and walk around listening back to the mix to check balance in the room, etc., but usually most of my actual "work" is done probably somewhere around 70-75dB or so, with some things going as low as 45-50dB.
G.