Automatically replace the formula in a column

How do I replace the formula in a column that had been overwritten with a number after that number is cleared?

See attached file. Cell B6 has simple formula of $C6 * $D6. I must allow data entry into cell B6 for an override. If cell B6 is cleared, how do I get my original formula located in column E6 back into B6.

I've tried locking and protecting cell B6 but then I can't overwrite it.
I figured out a way to do this by using 5 different fields, but this solution is really clumsy if I want to replicate Colum B. (see attached)
If must use a macro, I don't need the logic, I just need to know that a macro is required.

Thank you very much for your help.

Thank you sktneer.
Is the code mentioned in your reply code for a macro?
See I'm an analyst and not a programmer and I've never written a macro, so I tried copying the code you provided into a cell and all I got was text.