I have sent an email to verify my understanding, but while I'm waiting for a reply I will offer the following:
It's my understanding the NFIP policy will only pay up to the policy limitation. IOWs if the replacement value of the building is $1,000,000 and the RCBAP is only 80% of replacement value the RCBAP will only pay up to $800,000 for a total loss. What are you going to do for the other $200,000?
For the coverage amount refer to the following from page 42 of the Mandatory Purchase Guide.
(3) Policy Limits
The maximum amount of building coverage that can be purchased on a high-rise or low-rise condominium under the RCBAP is the replacement cost value of the building or the total number of units in the condominium building times $250,000, whichever is less.
Therefore if my assumption is correct then the 80% coverage will not meet the minimum coverage requirements under the LAW.
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The opinions expressed are mine and they are not to be taken as legal advice.