PF: Which page number should applicants enter in the Ethics explanation box in Part A to refer to Ethics in the proposal?

Page indication is to help the evaluators to navigate to the specific parts of the proposal where they can find relevant information, to speed up their assessment. In that sense it refers to any page of the proposal, where this information could be found. Information in Part B2 is easiest to find because it has a dedicated Section 6 Additional ethics information, so the page indication comes particularly handy while screening Part B1.