Need to create a transformation based on Update Rules Logic

Hi,

I have an existing complex Update Rule. I need to manually create a Transformation based on this Update Rule logic. The Start Routine of the Update Rule comprises of:

1) All data declarations in the Global Area

2) The local coding area consists of various select statements from various r/3 tables used later for mapping. It also calculates and stores values in the Data Package final internal table for few infoobjects that are not present in the Source object but and are used to populated data in the target.

3) then we have the various one to one individual Infoobject mappings/constants and many individual infoobject routines that pick result from the comm_structure

I am not very clear as to where each of the above coding logic should be put in the transformation coding area...... I can see four main coding areas in the transformation....global area, 2nd Global Area, Method Start_Routine and Method Inverse_Start_Routine........... I think the global data declarations(point 1 above) should be put in the Global declaration area of the start routine. The local area of the update rule logic (point 2 above) that contains select statements should be put in 2nd Global part or should it be put in Method Start_routine????? Point 3 above for individual filed mappings will be done through Rule groups. Also can anyone let me know what is the methos inverse_start_routine used for?????

Thanks.