PS.... I wrote:
To do __exactly__ as you describe, I would create a Worksheet_Calculate
event macro to update the counts in D1:D5.

Although that is what I would do based on your design and solution
limitations, I can offer one alternative that does not require a macro.

However, I do not recommend it.

This solution requires the use of one extra cell. I use F1, which is
initially empty.

Then enter the following formulas:

C3: =IF(F1="","",(C1-C2)*2/(C1+C2))
D1: =IF(C3="",0,D1+(C3=0.4))
D2: =IF(C3="",0,D2+(0.2<=C3)*(C3<0.4))
D3: =IF(C3="",0,D3+(-0.2<C3)*(C3<0.2))
D4: =IF(C3="",0,D4+(-0.4<C3)*(C3<=-0.2))
D5: =IF(C3="",0,D5+(C3<=-0.4))

Set the Iterative calculation option with Max Iterations set to 1.

When you are ready to start your experiment, set F1 to 1. That is the first
recalculations. Press F9 successively for subsequent recalculations.

Clear F1 to start the counters at zero again.

To set the Iterative calculation option:

1. In Excel 2003, click on Tools, Options, Calculation.
2. In Excel 2007, click on the Office Button, Excel Options, Formulas.
3. In Excel 2010, click on File, Options, Formulas.

Caveat: I deprecate the use of the Iterative calculation option for two
reasons. First, it is unreliable in some situations, although it is
probably reliable in this limited usage. Second, it masks mistaken circular
references; that is, you will not get an error message.

But again, I would do this experiment very differently altogether, not
relying on either a Calculate event macro or the iterative calculation
option.