Almost impossible to calculate exactly, you would need to know the increase in wing area with flaps down in my opinion. Assuming by some flook that the wing area stays the same, I make it 156.6kts...
knocking out the constants in the lift equation and dividing one by the other, I get (Vs)^2/(Vsf)^2=0.32. So Vsf (wings level stall speed with flaps as in situation here and weight 6400kg)=135.7kts
Increase load factor for turn increases stall speed by 15%, therefore stall speed is 156.6kts.
Allowing for the fact that we are not given wing area so would expect a slight error, I'd go for 146 as the nearest. I think your 18% is a bit off - note that CLmax doubles.