Your main breaker doesn't have to be the sum of all the downstream breakers. That would assume the each breaker were fully loaded all simultaneously. There's an allowance in the NEC for diversity. I recently added a new main breaker to serve my wife's new greenhouse and a 220V and additional 120V circuit for my garage. I put in a 50A main that feeds a 30A 220v breaker and two 20A 120v breakers, which would be 70A if you just added them up. Ran the install by my Electrical Engineer business partner.
