SAP BW Routine read Master data and set flag

Hello.

I load data from DSO1 in CUBE1.

In the loading data i have 0MATERIAL

in the cube i have the InfoObject 0MATERIAL and FLAG1

By loading the data from DSO1 into CUBE1 i want to check from 0MAT_PLANT with key 0PLANT = 1234 and key 0MATERIAL from DSO1 source the value from 0MAT_PLANT -> attribute Z1.

If the value in Z1 is 0 or emty make in FLAG1=1 else FLAG1=0

Where to implement the routine?

Start and/or Field and/or End Routine in the Transformation?

Please help me with the ABAP Code.