Checkboard,
Hmmm... I went and got interested in this, and then couldn't stop until I'd worked it out! Anyhow, your calculations do presuppose a fixed angle of bank.
According to the geometry of a banking aircraft:
v x v / r = g tan(AoB)
Your view of a flight computer shows a calculation in progress
v x v / r = 400
which indicates a fixed AoB (Angle of Bank).
But for slow aircraft sticking to a simple Rate 1 turn:
r = v / 188.5 using aviation units.
So the v / 200 suggested above will work reasonably well assuming the wind is not too strong.
[This message has been edited by Oktas8 (edited 25 April 2000).]