You can try this one,
Radius (ft) = Velocity (fps) squared divided by 32.2 x Tan of Bank angle
Typical flight directors limit max bank angle to 25 degrees, but most pilots use 1/2 bank at the speeds that you are talking about so the bank angle would be 12.5 degrees, so if you whip out a scientific calculator and figure the Tangent for 12.5 degrees (0.2217) and multiply your TAS in knots by 1.687 to get fps you can work up a quickie formula.
R (ft) = V x 1.687 sq / 7.139
for 280 knots I get 31,254 feet (5.9 sm)
and 400 knots = 63,784 feet (12 sm)