I'm picking up what you're laying down APC, but retro talks have historically only involved straight pay rate differential with no consideration given for other sundry items like work rules, sick time accrual, 401k matching, etc......

Pay disparity, of which sUA has a greater percentage, and time disparity, of which sCAL has an additional year since amenable date, are the primary driving factors the arbitrator will have to take into consideration in determining the split ratio.