if i understand ur question, u want to know when 50 percent of all units on a specific side are not present.
to do that make a trigger covering the whole map, and set activated by: side u want to count.
then in the conditions: true and on activation: trigAList = thislist; numbAList = count trigAList
then make another trigger:
activated by: side u want to count
condition: trigAList == (0.5 * numbAList)
on activation: whatever you want...