creating a percentage formula to put in widget

hi, i have a dataset like this     A  ,   B ,   C , territory        what i want to put in the widget is the following formula  sum (A *100/ (B -C)) 

                                                3,     2 ,     1   north

                                                5,     4,      0   west

 i want to show a widget that apply the formula for the total without considering territory and also one for every territory (like a group by territory)   

i tried to create a group on territory but then i got lost with reset / increment type  and evaluation time of widgets, especially when i try to show the total widget that shows the numbers of the last group.

thanks