I have only one observation per sample

The tables give values starting at 2, I am having this doubt what is the value I should use.
I have used n=2 for calculating the things. The only reason being it is closest to 1 (my value for n)
for my situation is this correct.

I am having times for a parts processed on a machine, so I have only one timing per product processed.
I have to calculate the mean SD and allied stuff. Plot control chart limits stuff. So I am stuck as to what value of n is to considered for ranges calculation etc.
Please respond as soon as possible.


Rohit