Change one parameter to update content

[disjoint]

Hi all,
I have been struggling all morning with the following issue (pls realize that I am an excel moron)

I have the following formula: =STDEV(C12:C31)*SQRT(252)
With C12:C31 representing an interval of 20
I want this to change to 1 month, 3 month etc... Just by changing the number 20 somewhere in my excel to 45, 70 etc... For example if I use 40 I would have: =STDEV(C12:C51)*SQRT(252)

How do I do that in excel?
(If this is unclear please let me know and I will try to re-phrase my issue)


Also I have another issue (less important this one, and more likely easy to solve)
I have one column that loads data automatically, and another column that picks that data from that column, instead of manually dragging down the column that calculates data, is there a way to automatically have this column adjust itself as we add numbers?


Thank you in advance for all your help!!

[windknife]

Try this,

=STDEV(OFFSET($C$12,0,0,40))*SQRT(252)

You can modify red character according to your purpose.

Hope this helps,
windknife