how to create a "lrunning" standard deviation (for Borda's substitution method)

Sorry, this does go on a bit but it's necessary to explain!

Column A1 to A11 consists of numbers, all of which are set to zero to begin with and may consist of any number, positive or negative, including zero, but not blank.

Column B1 to B5 is the average of
(B1) A1 & A3,
(B2) A3 & A5,
(B3) A5 & A7,
(B4) A7 & A9,
(B5) A9 & A11

Column C is the difference between
(C1) A2 & B1
(C2) A4 & B2
(C3) A6 & B3
(C4) A8 & B4
(C5) A10 & B5

Cell D1 is the standard deviation of column C. All that gives a calculation using Borda's method of substitution.

The problem is that having D1 set simply to Stdev(C1:C5) will take into account all the cells, whereas I only want it to take into account the differences that have been entered.

So if A1 to A3 have been entered, then the stdev should be C1. For A1 to A5, then stdev C1:C2 etc.

Using a count formula does not work as some values may be 0 anyway and they can't be blank.

I've tried using VBA to count the number of entries made and using that number, but can't get that to work either.

Anyone have any ideas how to go about this?

what about something like:
=STDEV(INDIRECT("C1:C"&ROUNDDOWN(COUNTIF(A1:A10,"<>0")/2,0)))

Thanks.

the problem is though that if there is a zero in the middle of the 11 numbers entered (lets say A5 is zero) it will only give the SD up to the point of that zero.

I think I may need to do VBA where a button is pressed each time a number is entered and use a count but I've no idea how to go about that either!