This is obviously not my area, but.. suppose target is 90db, and we:
measured L=96db, R=91db. Combines to about 96db. So in this case, I need -6db.
measured L=89d, R=84db. Combines to 89db. In this case, I need +1db
So in the above chart, below 0db will ± follow larger correction, and above...