Well I don't know the answer to your question, but in my opinion it does not matter too much.
However hard I have tried to do all the recommended things about avoiding blight I have never managed to avoid it any year when we get a lengthy period of warm humid weather in spring or early summer. But that doesn't happen every year, and in those years blight rarely seems to be an issue.
Now I am a tidy gardener and always try to dispose of diseased plant materials very carefully, but on an allotment field with many plots not everyone will be as careful, and so as far as potato blight is concerned my philosophy is that chance plays a big part.
Because it is a fungus disease and spread by spores in the air it can literally blow in from miles away. It seems more common in summer so I grow first and second earlies only, but otherwise I just trust to luck.