In my experience typical console pan laws use -3dB at center, based on -3db being 1/2 power.
JR
Most british mixers are based on -4.5. It makes much sense for music recording/mixing, because the acoustic pressure of studio monitors combine more than quadratically. -3dB would be OK for totally uncorrelated signals, which is obviously not the case.
I read a survey that had measures of various studios, that showed the combination factor to be about -1/-1.5 dB for two speakers driven each at -6. That's consistent with my findings when I did monitor set-up in the 80's.
Pan law at -3 results in a level increase in center position.
Even large concert sound systems combine more efficiently than the assumed 3dB loss.