If i understand you right:
1. Taxes for BP can be configured with Derived Flows:
IMG -> Financial Supply Chain Management -> Treasury and Risk Management -> Transaction Manager -> Securities -> Transaction Management -> Flow Types (Transaction) -> Derived Flows -> ...
1a. Define Derivation Procedures and Rules
Let's create Derivation Procedure.
Now we have to assign Rules for our Procedure
We select original flow and it's direction
Then we create Derived Tax Flow.
Here we can use:
If we use Percentage Rate, we need to define rate in Percentage rate field.
If Calculation Procedure - we have to define Calculation Procedure (see. 1b)
1b. Define Calculation Procedure for Derived Flows
Here we can define currency and min/mas amount.
Next step, to define amount limit (either min, or max) and calculatioins: Percentage rate, Fixed amount.
If you define Percentage rate calculation - define rate in Percentage rate field
If Fixed amount - define amount in Fixed amount field.
Now assign Calculation method in Derivation procedure (see, 1a)
1c. Define Rounding Rules - here you can define rounding rules.
2. SAP menu -> Accounting -> Financial Supply Chain Management -> Treasury and Risk Management -> Business Partners -> Special Functions -> Standing Instructions -> transaction TBI7 - Derived Flows
And the last step - assign derived procedure to Business partner using TBI7 transaction.
Select either transaction (100 Purchase), or product type (01A Stocks), or product category (Stocks), or Instrument (Security) and then assign Dervied Procedure
That's it, when you create Purchase of Stocks you will see 0308 tax flow near 0120 flow.