1.Press f1 on any field of screen costs
2.Press F9
u will get the screen number(In Screen data). Double click on it.
3.In PBO double click on module MODIFY_FIELDS_FOR_FLEX_DET
4.here u can find a form routine modify_fields which is in include LCONWF0H.
Still if u r confused, put a break point and run.
Regards
Sreekanth