I've managed to fix my problem in the trigger editor rather than the data editor. Thanks to everyone who has provided help.
I'm going to post a description of how I've done it when I have more time.
It allows computer controlled units to see units within a 120 degree arc in front of them within their sight range. It also prevents them from seeing units that they cannot path to in a direct line, thus giving a simulated effect of being able to "hide" behind doodads with pathing footprints.