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I would tend to put it differently to those that talk about eye level.
If the plane is perfectly flat and it is at exactly 90 degrees to the camera then the horizon line and vanishing point will be at the centre of the camera image.
However if the camera and plane are not at 90 degrees, due to the plane being tilted or the camera being tilted, then the horizon will be lower or higher. Similarly if the plane is curved, then the horizon will move.
You can play around in Photoshop's 3D layers moving
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Not as fancy as Dave's 3D stuff, but I hope this illustrates what I was saying:
Doc_Pit wrote
Guys, especially since this wasn't really a Photoshop question, I really appreciate your taking the time. Thanks very much.
Doc, Semi has just put me on to this thread. It's a subject close to my heart, so I don't know how I missed it. You really should join in with Dave's Something for the weekend challenges. They are a lot of fun, and I think everyone involved has learned a new trick or two.
Eye level will always coincide with the horizon and any perspective vanishing points, because the higher your viewpoint, then the further away the horizon. When using this when drawing or compositing, we have to consider the subject height relative to the imaginary camera's point of view.
I did this about six years ago in response to an art student posting to this forum looking for ideas for a city disaster. The raggedy man two identical brothers, all different sizes in the image, but their eyeline is right on the horizon because he is the same height as the camera view point. The feral dogs are shorter, so they appear below the horizon. This is single point perspective. The building sides facing the street taper to a single vanishing point on the horizon, but the sides facing us are square on both vertically and horizontally.
This is copied from part of a post I put up on the Something for the Weekend thread earlier today (my time). The Ernest Norling book is the definitive guide despite being 70 years old. As Scotty once told Captain Kirk, 'Ye cannae' change the laws of physics Jim', so it might stand for a good bit longer. But the real gold is in the Student Art Guide tutorials and videos. As I mention in the post, I learned a new trick or two from that.
As well as the links below, Steve Caplin is one of the go to guides for Photoshop perspective, but I think most of his freely available videos have been taken down, and can only be found on Lynda.com. The other major source are the Bert Monroy guides for his giant Times Square and Amsterdam illustrations, but here again, only on Lynda.com. Email me at trevor.dennis (at) xtra.co.nz, or send me a PM, and I'll give you a useful tip 
But keep at it. Perspective is key to drawing and illustration.
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I mentioned taking time out to brush up on perspective. That was completely rereading Ernest Norling's Perspective made Easy, and a more recently published Perspective Drawing Handbook Both freely available as PDF files. Note, when I say recent, PME was published in 1939, and PDH in 1964 (or thereabouts). The Student Art Guide is very good, and has easy to follow video tutorials (without sound).
https://www.studentartguide.com/articles/one-point-perspective-drawing
I actually learned some cool tips from that, like how to divide up using perspective. Really interesting stuff. This was all driven by my refusing to believe an apparent truism of single point perspective, that being that even if the vanishing point is offset on the horizon, lateral lines will still be parallel to the bottom of the canvas. I was convinced that offsetting the vanishing point, would turn it into two point perspective, but I couldn't work out where the second vanishing point would be. So that was two days to convince me that half a dozen lines were correctly orientated. Crazy, but these threads are all about learning.
One more guide worth a watch, but for different reasons, is this collaboration between illustrators Tyler Eldin, and Matt Laskowski. This is more about giving you some tools rather than how various perspective models work, and I learned some very cool tips from it, some of it completely unrelated. For instance, if you build a complex compound path, and want to save it as a custom path/shape, then setting the path option to Exclude overlapping, lets you merge without the interior paths disappearing. Making custom paths is a neat aid as you can quickly drag out and position as many as you need. That includes grids, and 360° radials that you can place on vanishing points. Being paths, they can live outside the canvas and extend onto it. You can FT to make bigger if needed. This is even better now we can control path colour, plus paths can be made very fine so as not to obscure your drawing.
Guides can also live outside the canvas of course, which lets us set up a two point perspective like this. If you want to mirror the vanishing points, start with a centre guide, and zero the grid to it. You can then place the two vanishing point guides using the rulers.
You can see I have used a slight variation of Matt's radial perspective lines. It gets very busy at the centre, which makes precise positioning difficult, so knocking out the centre and placing crosshairs in it, takes care of this. This is another application of Exclude Overlap when merging. The default would erase the crosshairs. You need the circle btw.
One last tip that occurred to me when reading through the comments. Someone asked about removing the sketch lines after doing the final line work. The poster had missed the point of using layers, but if you did find yourself in that position, as well as the brush tool set to overlay trick, you could also set the layer to Hard Light which would knock out the mid tones. I suspect that would clean up the layer in a single stroke, so to speak.
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