I don't have a specific formula to answer your question, but there will be leakage regardless of size. This can be partially mitigated with metal screens or fully mitigated part time using steel scroll shutters or steel door shutters. Steel scroll shutters have the benefit of adding R/U insulation rating in cold weather and in some cases can improve physical security.
We want to be able to get natural light and air into the house. I can make a safe room if I need to, but this is an always-on threat and we can't respond with whole house unhealthy environments. Grounded metal (copper or aluminum) screens should work quite well if designed for the application I would think. But that means 100% screening, no fully transparent openings. Maybe we can come up with fluid filled windows that provide the required shielding. What a PITA.
