Stop for loop at certain condition

Here's some sample code for column C:

This works for the same number of cells as used in column A - up to the page break:

Sub test()

Application.Calculation = xlCalculationManual

Sheets("Sheet1").Select

    For Each r In Range("C1:C" & Cells(Rows.Count, "A").End(xlUp).Row)
        If r.Rows.PageBreak <> xlNone Then
            MsgBox "page break on " & r.Row
            Exit For
        End If
        r.FormulaR1C1 = "=(RC[-2]+RC[-1])*2"
    Next
    
Application.Calculation = xlCalculationAutomatic

End Sub

This one sounds like what you are after:

Sub test()

Application.Calculation = xlCalculationManual

Sheets("Sheet1").Select

    For Each r In Range("C:C")
        If r.Rows.PageBreak <> xlNone Then
            MsgBox "page break on " & r.Row
            Exit For
        End If
        r.FormulaR1C1 = "=(RC[-2]+RC[-1])*2"
    Next
    
Application.Calculation = xlCalculationAutomatic

End Sub

You can duplicate the main part of the code changing the "Range" to the appropriate column and the formula to match what you are after.

BTW You can comment out or remove the message box, I just used it to show that it had "found" a page break.