Override the data in the cell when checkbox is clicked

Hi
I need some help on overidding the data via checkbox.

Basically i am working on a spreadsheet that is doing two things 1) its allowing the user to choose between 1 to 3 number or N/A from the list validation option and 2) there is a checkbox if one selects that all the columns that have the formula will become N/A irrespect of data in it.

I am using the following formula in the list validation option
a3 is the check boc
E5 =1
E6= 2
E7 = 3
E8=N/A

=if(a3=true, $E$8, $E$5:$E$8)

The issue I am having with this is, if I start selecting something from the drop down and half way I realize I need to check the box, it will only make it E8 value for the cells I have not touched and the ones I have already selected the drop down value it will remain. However I want it to override the value to E8 irrespect.

1) If the user of the spreadsheet clicks on the check box (this is in A3 cell) which i have linked it using format control then all the cell (Column E) that have the formula =if(A3=True, "N/A", ) will have N/A which is not applicable and not an error as you mentioned. I have successfully implemented this

2) If the user does not select the check box (a3) then each of the cell (Column E) has a drop down which the user can select either 1,2,3 or N/A. Which I have successfully implemented by using the data validations

Now the struggle I am having is as follows: I have attached the spreadsheet for your reference too.

Now if I select number 2 in cell E10, 2 in cell E11 and 3 in cell E 13 and you can see alll the other cell in column E are 0, because of the formula and the box in A3 is not checked in Tab 1. In Tab 2 I have checked the box everything turned N/A in column E except cells E10, E11 and E13 because I had previously selected 2,2,3 respectively.
This is not what I want, what I want is when I check that box everything should turn to N/A basically an override function is what I want to created.