Since PB time walk is fixed arm-by-arm, we try to fix the T0-PA time walk more accurately, by studying the center and width of
DeltaT''(T0->PA) = Tpa' - Tt0' - TOFkstop - TOFsec',
where Tpa', Tt0' are 1st-stage-offset-subtracted Tpa/Tt0 (see, 0624), and TOFsec' is calculated from the updated PA-PB-detected 1/beta (see, 0627).