Yep, line of sight is determined by how a deer sees and cover. Consider those factors, and make a decision.
As mentioned, deer eye anatomy suggests horizontal peripheral vision is maximized, something like 310deg, which makes sense for detecting the majority of their predators. I don't know of any testing on the vertical range of deer vision, it'd be an interesting study, but it's not an assumption to say sight in the vertical axis is not the primary anatomical design of a deer's eye.
I know assigning deer senses human context is bad practice, but in this case even if deer had an advantage, the physics of the eyes is the same. So, lets consider human eyesight in the vertical axis.
The further an object is, the more of it we can see and the less detail, until it's beyond our visual range. It's like our eyes collect data from two cones of perception extending from our eyes that our brain combines and interprets.
What does that mean for the hunter?
The flatter the ground and thicker the timber, the lower you can be and still avoid a deer's vision.
That said, I'd focus more on what a deer can smell than see. They have like 20/100 vision but smell like 1000 times better than a human.
Line of sight changes when a deer tilts it's head to see what it's smelled.