I hope this isn't a homework question lol.
I'm also not quite sure what is being asked unless clarified.
Maybe another view point might help:
Let's call flying box -> fbox
We divide fbox into simple lengths 'a', so we can use diagonals to draw equal lengths in perspective.
Reflection along the surface is described by the orthographic side view.
It describes the reality of the lengths.
In the projection of what we see, the lengths of 'a' are distorted by perspective.
But by using diagonals, we can create lengths of 'a' which are in proper perspective.
For a reflection, we use fbox to draw a new rectangular box with a height = 2 * (fbox_height + fbox_distance_from_surface)
The surface we are reflecting fbox onto will intersect the middle of this new rectangular box.
If one is not convinced, look at the orthographic side view and think about it.
Below I have attached a drawing that I hope demonstrates the concept.
They are wonky since it was quickly done by freehand and linetool, but should be correct with a grid.
Again, orthographic view describes the reality of the length.
In the projection, the lengths will be distorted by perspective.
Details on how diagonals work and more can be found in 'Perspective Made Easy' by Ernest R. Norling
I highly recommend the book because perspective is made easy.
Another book of interest is 'Successful Drawing' by Andrew Loomis
Feel free to ask questions or request clarification.
I hope this helps answer your questions.