Try creating invisible neutral units (with a large sight radius) at regular intervals in the area of the cone in front of your unit and testing whether the neutral unit has sight of your unit. This should allow you to determine the blind spots in the cone. Then for the units that have vision, replace them with a small visibilty modifier to fill the visible portions of the cone.
@Shouchen: Go
Try creating invisible neutral units (with a large sight radius) at regular intervals in the area of the cone in front of your unit and testing whether the neutral unit has sight of your unit. This should allow you to determine the blind spots in the cone. Then for the units that have vision, replace them with a small visibilty modifier to fill the visible portions of the cone.