Try booking IST-SIN-CCU in one hit online, it will tell you backtracking. The chart indicates this can be the case (quote below). The distance is almost double (1.97x) the direct distance between IST and CCU. Remember the IST zone also includes JNB, and JNB-SIN-India bookings almost certainly would not be considered backtracking.
Interestingly IST-SIN-CMB does come up. It is only 1.82x IST-CMB. Does this mean the magical number that separates backtracking and non-backtracking itineraries on the SQ table is somewhere between 82 and 97% of great circle distance?
Zonal combination includes certain backtracking routes to which the reference fare will not apply. To confirm the exact mileage required, contact KrisFlyer Membership Services.