Both you and Maggie haven't quite got this right. Herd immunity for measles requires at least 92 in every hundred people to be immune. We've had outbreaks in the UK when the number has dropped into the 80s. Fortunately, immunity is long-lived. The threshold for flu is much lower. For certain childhood diseases, such as whooping cough, the overall population threshold isn't a great way of looking at it: it's far more important that children and young parents have the highest percentages, as they overwhelmingly are the children's most frequent contacts. Another factor is that herd immunity can diminish over a short time if (as we might suspect for coronavirus, though we don't yet know) immunity is short-lived or lost as people age with a consequent weakening of their immune system. I wouldn't be hanging your hopes on that 70% number if I were you. That's about the median figure of the estimates, but that figure may well be confounded by interventions such as lockdowns and shielding, both of which, for very good reasons, artificially keep the transmission rate down, thereby extending the time it will take for herd immunity to kick in.