Here's in-depth info on solar flares. Major ones are considered "Carrington-level events", due to a flare that occurred in 1859: https://www.history.com/news/a-perfect-solar-superstorm-the-...
Vox's 2014 breakdown of the systemic risks: https://www.vox.com/platform/amp/2014/7/30/5951263/a-catastr...
NASA article on the prediction that there was a 12% chance a Carrington-level event would occur by 2022: https://science.nasa.gov/science-news/science-at-nasa/2014/2...
My 40% is based on that information. I'm gut-extrapolating to the next few decades, with the understanding that large devastating flares occur every few hundred years.