I know this request may be "pie in the sky" thinking, but I thought I would
present it to the incredible mind trust on this forum.

I have a range of cells that calculate values based upon a formula.
Sometimes I want to override the formula and manually enter data. However,
if I ever want to go back I have to copy or insert the formula back in. Is
there any way for a cell to automatically detect when there is a null entry
in an individual cell and in that case place a pre-defined formula in the
cell instead?