Formula to get data from 7th field (dynamic entry)

Hi guys,

I'm trying to do a simple calculation where I minus two cells with one another to get the sum of the difference.
The catch is though, the cells are in a column that is ever-increasing with new entries. I am trying to do a %Change calculation on a 7 day basis. So in other words, I'm trying to calculate the %change of the 7th-last entry and the last entered entry in the column (this moves every time as new data is added to the bottom of column A, and needs to update with the most current values, but it is always the 7th-last entry minus the last entry of column A.

I have attached an example of my sheet. The area of interest is marked in red.
Yellow represents the constant stream of new data that is added.

Any help would be much appreciated