You can use any good faith determination of value. Hazard insurance (although it can be like comparing apples to oranges), replacement cost, actual cash value, tax assessment, etc.
You say the deductible is $2,500. The NFIP deductibles are as follows:
Pre-FIRM (Flood Insurance Rate Map):
A.) Flood Insurance equal to or less than $100,000.00 = $1,500.00
B.) Flood Insurance greater than $100,000.00 = $2,000.00
Post-FIRM (Flood Insurance Rate Map):
A.) Flood Insurance equal to or less than $100,000.00 = $1,000.00
B.) Flood Insurance greater than $100,000.00 = $1,250.00
Is the agent using private insurance that has a higher deducible than the NFIP's?