To clarify a little further, the "Optimal Transmission Zone" is a vertical 60° angle between the controller antennae and the drone. As long as you maintain that angle, you will be in that zone, but obstacles could drastically reduce the horizontal range of the zone.
The critical component of the zone is the attitude of the antennae in relation to drone. You can have the antennae folded perpendicular (straight back) to the controller, but if your controller is pointed downwards toward the ground, your OTZ will be lower to the ground.
As you can see from the above images, the controllers antennae are not folded straight back, they are at a 45° on the controller which is fine if that is how you hold your controller, and some people do. If you hold your controller more flatly, you will need to fold your antennae further back to make up for the flatness of the angle of the controller.
In diagram A above, antennae orientation is aimed too high, and the drone will be out of the zone. The pilot could either point the controller further down or orient the antennae that way. In diagram B, it is aimed too low, and the pilot can either fold the antennae back against the controller, and/or rotate the controller back to get back into the zone.
Im sure many of you know this already, and as
@ksmusa has said, make sure you are pointed toward the drone. If the drone is behind you, so is the zone.