I've fixed the correction factors and extended the column up to 80C. (I got a few values slightly wrong the first time.)
The correction factors turn out to be the same on both sides of the pond. I found the factors had been calculated using this simple formula:
factor = root( (Temperature rating of conductor - actual ambient temperature) / (Temperature rating of conductor - reference ambient temperature) )
If you have a table with ampacities at 30C but the actual ambient is 40C and the conductor is good for 60C you calculate it as:
root((60-40)/(60-30)) = 0.82
