Whenever a formula contains an item of null value, the formula always gives a result of .NULL. except that the null value is the argument of the function IsNull( ) or the first argument of NVL( ).
For example, if you write a formula:
14 - Column('Ave FP')
and the value of the item Column('Ave FP') is .NULL., the result yielded from the formula will be .NULL.
i.e. 14 - .NULL. = .NULL.