It's just the arrival horizontal angle of the sound, in other words the inverse function (actually an approximation, because I believe the inverse function can't be expressed with elemetary functions) of function ITD = r * (alpha + sin(alpha)) / c, where alpha is the angle of the sound in radians, r is the radius of a ball approximating the head and c is the speed of sound. I use values r = 0.085 m and c = 345 m/s, which is pretty accurately the speed of sound in my room temperature (the common value 343 m/s is for 20°C/68°F, but my room is a few degrees warmer than that).
It seems that our hearing adapts to constant ILD shift a bit. If I listen to music with headphones so that right channel is boosted a couple of dBs for several minutes, it starts to sound somewhat centered and if I then remove the boost, the sound moves to left, but returns to center in a few minutes. Maybe it's just me who has experienced this. We should never evaluate the changes immediately after a change, but after a minute or so to let adaptation take place. Same with crossfeed. Exposing our ears with excessive ILD makes our hearinf adapt to excessive ILD and then when we put crossfeed on, the soundstage seems to collapse narrow at first, but after a minute or so, it has expanded back when our hearing has adapted to more natural levels of ILD. Kind of Motion-induced Blindness for spatial hearing. I believe the problem is the lack of supporting information from other senses. When you walk from a room to another room with different acoustics and size, our eyes inform our brain about the change in acoustic space and our hearing knows the spatial information will chance accordingly. When we adjust sound listened with headphones, our eyes don't register chance in acoustic environment, but the sound chances. It confuses our hearing and adaptation time is needed. I realized this when listening to my own binaural recordings with headphones. I had recorded coming home from out including several changes in acoustics. Those changes sounds VERY dramatic when listening to the recording, because I am sitting still in my room and my eyes tell me the acoustics should not chance but it does. In real life anyone hardly even notices the changes because they are expected and logical. A lot of stuff we "hear" is actually our brain/senses fooling us rather than our soundgear doing strange things.