Unless I,m missing something, which is highly likely
If RS2 is incorrect when positioned into place , then , surely RS1 will be too??
I haven,t got my book at home with me but if you have needed to shorten RS2 then it would appear that BR12 is not "true", if you have only need to shorten RS2 & NOT RS1 then the fault lies in the BR rails.
As others have said , the diff cage is basically a TRUE box welded to the back of the chassis , remembering that you need to leave a 3mm gap for where the small plates are fitted on the cockpit side of the seat back area.
RS10 should be at right angles.
Be careful when presuming all is well by shortening parts here & there to get your measurements correct, ( shouldn,t need to ).
This will cause you problems when you come to fit the wishbone brackets to the diff cage if the diff cage is "wonky" as the placement of the wishbone brackets are measured from various points including the rear face of BR12.
If I had my book I could be more precise
cheers
andy