I didn't say that.
Say you want to do this for all of the east units in your mission. Create a large trigger to cover them all set it to east present and in the activation field put
AllEast = + thislist
You have now created an array of all the east units at the start of the mission (neglect to put in the + sign and this list will constantly update as units die, with the + sign it is frozen for the duration of the mission.)
Then in the trigger you refer to above have the condition
({alive _x} count thislist) < 0.5* (count AllEast)
I have not tested this and when I want to do things like this I use scripts, but the above should give you some idea of the sort of thing to do.
NB:
If it works - do an experiment. Take out the + in + thislist, and I predict it will stop working.