Hi,
The definition of "Define Derivation Procedures and Rules" for derived flows are only taken into account during transaction management, deriving one flow type from origin flow type. This customizing does not have effect for FWBS. You may check the below customizing:
1) Transaction Manager > Securities > Master Data > Product Types > Define Company Code-Dependent Settings for the Product Type: mark the indicator "Generate taxes" in Settings for tax generation.
2) Transaction Manager > Securities > Position Management > Securities Account Management > Define Tax Rates.
Further you may consider the implementation of BAdI TPM_TAX_CALCULATION (please see note 1159408) to calculate the tax depending on your requirements. Note: the above BAdI is called only in the case the required settings are done in 'Define Tax Rates' in the Customizing.
Best regards,
Jasmine