FSG - calculating DSO

Hello,

I am trying to get the DSO Calculation using FSG,  and it is really my first time, and this is my concern, Y5  is the last 3 months sum , But I just want this Calculation for ( X1:Y5), but  as you can see for (X2:Y5)  also has the same calculation, I would like to have this in blank.

Also I need to add a row for the DSO calculation that will be (X1:Y5) / (X2:Y4)

                                                                          (Y1)                             (Y2)                     (Y3)                  (Y4)                              (Y5)

                                                                         Current Month            Month 1             Month 2          Last 3 months SUM       Average Days of Sale                                    
                                                                           --------------              ---------------     ----------------     ---------------------              --------------                                          
(X1)Trade Revenue                                              -748,084.80         -934,191.86         -885,288.61       -2,567,565.27              -28,528.50