When making denatured alcohol in accordance with a formulation specified in paragraph 1, 2 or 3, water may be mixed with the alcohol before denaturing or with the denatured alcohol but the quantity of water added must not reduce the proportion or quantity of denaturing substances F1... in the resulting mixture below the proportions or quantities specified in the formulation; and for the purpose of ascertaining the proportion or quantity of denaturing substances F1... in any such mixture the water shall be treated as if it were alcohol.