I think a base should start flashing as soon as shells start exploding near by or the fleet is visible on the horizon (what range is that?).
But apart from that, missions do often have to wait until the last moment because LVTs need to spawn at roughly the same time, and for that the fleet needs to be close in shore.
Maybe all flights could be prevented from a CV for the 5 mins before take-off (although that's open to abuse by changing the t/o time).
Tricky one.