Hi
Why do you do a manual accrual? Think you can have an scale, but you need to account the accruals according an average rate.
But I suggest to see these notes as checklists for your issue
SAP Note 456247 - Accrual conditions w/ calculation type 'B' - fixed amount
SAP Note 410579 - FAQ: Rebate processing
SAP Note 105681 - Consulting: New rebate procedure
SAP Note 456458 - FAQ: How does SDBONT06 work?
SAP Note 613089 - Enhancements for the use of 'Enhanced rebate procedure'
SAP Note 609631 - BAdI for use of 'Enhanced rebate processing
SAP Note 7 5778 - Consulting/troubleshooting for rebate processing
and related notes. Maybe someone gives you an idea how to solve your trouble.
I hope this helps you
Regards
Eduardo