using content from the file u made, if you would try to set an entry value on page P0 to CSV.At("a","c", "P0") & " + " & CSV.At("a","c", "P1") then as far as i can remember CSV.At("a","c", "P1") will always return 0 or stay empty, or it will turn the page to P1 and will try to set value over there.
EDIT@ so maybe you could add PAGE to SET VALUE, so we would have the following: PAGE,COL,ROW,VALUE?