The 30 day grace period after expiration covers the property, so in your example the flood on the 25th would be ok. The grace period is to allow time for the policy holder to pay the renewal premium.
The gap between days 30 and 45 is your concern. If the property were to flood on one of those days, there is no insurance coverage. This is the reason many banks either have a GAP policy for the interim before forceplacing, or go ahead and force during the notification period and EAT the premium for those days.
I can't herd the cats anymore, so I just set up the electric fences and let them fry when they stray out of bounds.