For this scenario,
First find the last cell Example: C20,(it is the bottom right last cell in excel file)
based on this value C20, try to loop the values from C20-C10 and within that loop use variable to operation and put those values in List variable and perform average operation.
NB: C20 is cell index and decrease the C20 to C10 by making it dynamic, i.e. if other file it is C50, then it should loop from C50-C40 likewise move on.