Multi city tool forces 2 hours between connections, regardless of MCT.

1h 15 should be adequate for a T5-T5 connection. You’ll have to clear immigration and security coming in from Zurich; the eGates have made a huge difference on INT-DOM transfers and security feeds into south security which is generally OK outside the morning peak. If you miss it BA will put you on next available flight - I am assuming there is a later one to MAN?