drop down validation with formula locked if deselected?

[vause]

I want to allow drop down based on validation, & also have cell formula coded to calculate based on other drop downs if selected first (output is valid). Want to allow user to select drop down BUT if deselect have formula locked in cell & not deleted. Any ideas? (have tried macro to disallow blank but it doesn't save formula from deletion, just mandates valid selection from drop down)

Example:
Lvl0 is in C2; there are 6 valid entries (5 specific + "select one") which are coded in data validation. Lvl1 is D2, Lvl2 is E2 and Lvl3 is F2. The content of C2 is coded such that is auto fills (from the valid entries) based on the contents of D2,E2 or F2. I want someone to be able to override formula to select a specific valid field but if they select "Select One" or try to leave blank, the formula remains in the cell.

Please let me know if this is unclear. I tried a quick macro disallowing a blank entry, once A2 is selected it only demands that a valid entry be selected (that the field not be blank or have other entry) but the formula is gone regardless of selection.

THANK YOU!

(ETA - Cross Posted at: http://www.mrexcel.com/forum/showthread.php?t=517564 - unresolved. newbie apologies)