SAP XI - BAPI routing

Hello,

I have a scenario as below, can you guys pls help me out

1)Data is extracted from a flat file through a SAP-ABAP Proxy and sent to SAP XI/PI.

2) In Xi/PI , depending on one string length the BAPI needs to be rounted to 4.6 C or ECC 6.

3) If String lenght > 6, Call BAPI in 4.6C else call BAPI in ECC 6.

Stpes I have followed:

1) Created a document,Message Type for flat file data from ABAP proxy

2) Created a temporary document with the one value as STRLEN.

3) Created a Mapping between MT for steps 1 and 2.

4) In the mapping , I have used the "lenght" function and mapped to the STRLEN value in temp document.

I am not sure from here how to proceed further , can you pls help me out in completing the steps

Thanks in advacne