Black-Dawn,
That may be but your rule does not hold true for all cases.
BEagle has it exactly right with the Nm per min squared divided by 8.89 and his practical application of rounding the divisor up to 9 still gives a near result.
Not all contributors to the forum operate only on heavy jets.
Using BEagles rule for your 420 TAS example yields a result of 5.44nm turn radius against an actual of 5.51nm which is closer than your method and has the virtue of applicability at all speeds.
[This message has been edited by The Vicar (edited 07 June 2001).]