The 'play-it-by-ear' limits we used on S&R were tempered by experiences gained/endured by our fellow operators. The Thorney 4-in-a-day callouts which were conducted throughout on Decca and eye-balls-on-stalks lookout, or the Leconfield tragedy of the Whirlwind attempting to climb out of a 'valley trap' and entering a Cb, leaving in pieces at 12000'. While you can train the techniques for S&R, each situation is different and is dealt with individually.