Hello, Grogoriy!
Thank you for your answer and topic.
Can you help me, How can I write that interest is charged on the amount of the transfer, without giving a specific amount?
The rules of derivation I indicated flows 1106 (borrowing) and 1914 (Charges), but do not quite understand how it will work then. In general, I have a reward as the main flow, I've yet to create rules for derivation derivative flow 1914 "charges". 
Assign calculation procedure to BP.
In derivatives transaction flows are specified only on the "Other flows", which also need to specify the amount.
And I do not understand how it works
Many thanks,
Regards,
Ainura