Hello BOAC! You're not RTFQ! 644 + 1.2T (T=deg C) gives ISA SL SOS 644+18= 662kts.
At -40C, SOS=644 + (-48)=596kts (or 594 kts by your method which involves dividing square roots by square roots and multiplying by 660- easy upside down in a Canberra if you're good at mental calcs!).
I think a simple mental calculation using a basic easy formula accurate within better than 1/3% might be the better option!