1. You can rely upon a previous SFHDF if 3 conditions are met:
a. The determination was document on a SFHDF,
b. It's still accurate (there have been no map changes)
c. The determination was < 7 years ago.
The 7 years is measured from the date of the determination. Therefore, a SFHDF dated 1/1/10 can be used for subsequent loans until 12/31/16 assuming the other 2 factors are met.
2. To determine the proper amount of flood insurance, you must consider 1) the insurable value of all collateral in a SFHA tied to each loan, 2) the loan amount and 3) the maximum amount available by the NFIP per building. If you have multiple loans, you would do this for each loan, but cannot exceed the NFIP maximum per building.
Rather than provide arbitrary examples, it would be better if you posted specific examples in the Flood Forum. Most of these require lots of explanations as there are so many variables to consider.