Como calcular un campo TOTAL en GRUOP BY

As I can calculate two columns fields TOTAL GROUP BY as follows:

    CAMPO 1       CAMPO 2       CAMPO 3
       1000           1500          150   
       2000           2500          125

TOTALES: 3000 4000 133.33

field calculation 3 is (Field 2 / field 1) * 100, I have no problem with that because I do in the event of onrecord
what I need is how I can calculate the total “field 3” which is (Field 2 / field 1) * 100 which is the same calculation of onrecord but now need to do the same calculation in cuts GROUP BY TOTAL fields as indicated above.

I welcome your support.
Thank you.