Yes, use FL for calculating your sales tax deduction. Using FL as your basis for calculating a U.S. sales tax deduction is consistent with the intent of the U.S. tax code.
As an active-duty military member, your income is treated as earned in FL and you should treat any income-related deductions (i.e. sales tax) as if they were also sourced in FL.
The IRS doesn't have any way of calculating the U.S. equivalent of the German sales tax (VAT) - their tax rates and currency cannot be easily translated to a U.S. equivalent sales tax.
