Panning Law (Equation)?

Audition is a competative commercial product, with all the non-disclosure arrangements you'd expect in place. No formulae for any part of its operational or analytical functions has ever been released, nor will it ever be.  Nor, despite what you claim, does this affect its reliability in any way. Audition is used for editing audio, and as such the information you are after is irrelevant to end users who use their ears to determine where a signal should be panned to in a sound field.

The panning law is not fixed at -3dB/center - you can vary that value. And it's not the only panning option either - there's a L/R cut (logarithmic) option too. But regardless of that, the amount of pan at any given setting would be easy to determine using some test tones. If you are really that desperate to know, then I suggest that you do a little research along those lines.