This is something that's confused me immensely for quite a while, and I've seen that a lot of people have felt the same way. Since I don't deal with plotted perspective much, I've mostly just ignored it but it has had some negative impacts on my work in the past. So, here's the results of some research I've done into the subject. In looking into how to properly construct ellipses inside of planes, I also discovered some neat tricks that allow us to easily construct perfect cubes in space a very handy tool for building up grids and doing measurements.
Drawabox is a series of free structured drawing lessons that explore the basic mechanics of drawing, along with the fundamental skills of capturing the illusion of solid form and constructing complex objects from their simplest components.
You can find more free lessons at https://drawabox.com
Other Links:
/r/ArtFundamentals community on Reddit: / artfundamentals
Drawabox Discord Server: / discord
Patreon: / uncomfortable
Facebook Page: / drawabox