Thanks for the info.
Cosmo, the circling is based on an Angle of Bank of 20 degrees. Ref Jepp Text pg. 33. Plus I think you've made an error in the calculation, you used thousands of meters per minute instead of meters per second. I get a radius of 1,522m at 180knts/30 Aob.
So you think it's more to do with keeping the runway in sight? Fair enough, do you have a reference to where that might be specified?
If you had to circle on a day with a low cloud base but great visibility, wouldn't it make sense to extend downwind a bit to give yourself more space?