[SOLVED] Last 6 data for std deviation calc

Say, I have a row of 10 data. I calculate the std deviation using stdev function on the 5th, 6th, 7th, 8th, 9th and 10th data.

A month later I add 11th data to that row. How can I set it up such that the stdev will automatically select the LAST 6 data (ie 6th, 7th, 8th, 9th, 10th and 11th data)?

Thanks.