You might want to check out the Statistical > Col Mean() formula. Create a new column with a formula ColMean(Raw_Value, test_name, measurement_name, entity). This will give the same means as the summary option but repeated across all rows.
If you order your table by date (decending), along with the other options you can create a formula which increments a counter and stops if the previous row and current row have different entity labels, etc.
You could then create a new column which copies the Raw_Value only if the counter is in the range 1-13. If you alter your first formula to point at this column, it will only take the means of these 13 or fewer values by entity.