Derived answers are location-sensitive.
Derived answers in the QAT can use info above them in the QAT and can use info contained in Grids.
Derived columns in Grids can use info in the QAT and Grids that precede them and columns that precede them.
Phrased another way, these are the no-no's:
Derived answers in the QAT cannot use info that appears later in the QAT.
Derived columns in Grids cannot use info in columns to the right of that column.
Derived columns in Grids cannot use info in later Grids.
Or, in plainer English:
Derived answers can only refer to answers that precede them, with one exception: derived answers in the QAT can refer to Grid answers.