formula auto input
 

I have a work sheet with several data validation drop down lists, each cell
also has a formula to input data from another cell. If the formula returns a
blank cell which is often, the user will select from the drop down list to
add data.. Once this is done the formula is then deleted from the cell which
is fine. If the user wants to return to a blank cell they can just delete
what ever is in the cell this is also fine. What i'm trying to achieve is
this: When deleting data returning to a blank cell the original formula be
auto re inserted into the cell! Is there a code or event code that i can use
to achieve this?

Any help is appreciated

Regards