Do we know the probability distribution for annualized real total returns calculated over rolling 30 year historical periods for a balanced 60/40 fund?

Google AI is telling me it can't find a standard deviation for rolling 30 year performance and the standard deviation wouldn't help anyway because the tails are fat so not a normal distribution. It did tell me, when I asked, that it has never been less than 50 percent of the mean. I don't mind running online backtest models but if we know the 85 percent probability of exceedence value it would sure simplify things.