Hi,
I'm not sure how have you created your BRF+ condition.
But in general for TCM rules it is always good to go via TM conditions UI, it can be located by path
Application Administration-->General Settings-->Conditions-->Create Condition.
Create a condition of "TCM_AMNTFR" condition type where you can write your logic to derive an amount. This condition can be assigned to your charge line in TCCS either directly in "Direct Formula Rule" field or via "Rules for Charge Calculation by Formula" as described earlier.
Thanks & Regards,
Tarun Kumar