using distinct function

using distinct function

I have a blank multiselect field called ' WO subtotals ' (WO short for Work Orders)
I have  subform Containing a WO Group field and an Est. Hours Field  and  a Create New Group Field in each row
 
starting with a value of 1 in the 1st row in the  WO Group field each new row will be also be 1 until I select 'Yes" in the  Create New Group field of a given row, then that row along with all succeeding rows becomes 2  until I press it again, then it become 3 and so on

I want the sum of  Est. Hours  for each group to display in the  WO subtotals multiselct field.

so If the row 1-3 were all  WO Group 1 with row  1 est hours being 3, row 2 being 4 and row 3 being 1, the total est hours for   WO Group 1 would be 8.

Any help is appreciated