Mid point b/w (18,-6),(0,0)
X = 0+18/2 = 9
y = 0-6/2 = -3
thus mid point is (9,-3)
Calculate rank percentile in Excel
To calculate the rank percentile of a list data, you can use a formula. Select a blank cell that you will place the rank percentile at, type this formula =RANK. EQ(B2,$B$2:$B$9,1)/COUNT($B$2:$B$9), press Enter key and drag fill handle down to calculate all rank percentiles.
I think it would be 12.7, i am not completely sure though.