Conditional formating an empty cell based off of another cell

Hello !

I am currently working on a worksheet that has 2 tables for two distinct equations that are used in 2 distinct cases.

Case 1 uses Equation 1 that has 9 variables

Case 2 uses Equation 2 that has 7 variables

In the E column I inserted Text that dictates if I use Equation 1 or Equation 2. The variables for Equation 1 are written on F:N, and the variables for Equation 2 are written on O:U.

Now, I want to write a conditional formating for cells that adds "N/A" to the cells that aren't currently in use by the Case in question.

So, if for the row 4 I have in the E collumn "Case 1", I want cells F4:N4 to remain blank and to be able to write in manually the data, while cells O4:U4 to have the "N/A" notation since they're not in use.

The issue is that excel won't add the "N/A" notation unless something is already written in the cells. I want to make it write "N/A" even when the cell is empty.

I use excem 2010.