A little rant about percentages

Sorry to go a bit off-piste, but I am going to talk about the weather. The Independent reported the Met Office saying that this has been the coldest spring for 50 years. It is a story we are all interested in as we continue to shiver in our thermals.
The paper gives some figures - that the average temperature for the past three months has been 6.0C which is 1.8C below average. This, it says, means that the temperature has been nearly 25% below average. My first thought was that it was more than 25%, since 1.8C is more than 25% of 6.0. But they had been careful, and worked out that 1.8C is nearly 25% of 7.8C. But they have missed a larger point. Zero Centigrade is an arbitrary point - it is for instance different to zero Farenheit. It is 273 degrees above absolute zero. So I suppose if you wanted a percentage figure it would have to be 1.8/ 280.8 - which is definitely not anywhere near 25%.
There are two points to learn. One is the problem that so many people who are skilled with words in this country are innumerate and technically ignorant. The other is that the construction industry, which loves to bandy round statistics with little basis, needs to be careful. Increasingly there is a desire for measurable benefits to justify expenditure. If the basis for these figures proves to be flawed, it may result in a significant setback and loss of confidence.