I need to to subtract two cells. If the cell is empty I need a null return.
e.g.
A1 B1 C1
22 20 2
10 8 -2
10 The value in C1 needs to be blank, so that when the value in A1
is added later then C1 will recalculate.

The resultant calculated values will be used in mean, standard deviation and
COUNTIF calculations.
As always many thanks to all of you out there - you are doing a brilliant
job for duffers like me.