You mesure the cone of vision from the station point. A 60 degree cone of vision is a 60 degree angle (30 degrees on each side) that you extend to the horizon line. Using the center vanishing point (CVP) draw a circle which will be your cone of vision. I drew an example of a 60 degree cone of vision, but it’s basically the same thing for a 90 degree one (it’s just a 90 degree from the station point, 45 degrees on each side). Hope this helps!

Mileage may vary with this advice, but I like doing comics as projects to get better at just about everything. A comic book page offers up challenge after challenge to solve. As you solve those problems, you will level up--page by page, and panel by panel. I've seen storyboard artists recommend this, as well as animators. I make t-shirts for a living, but making comics is where I find I improve the most. If you want to improve on perspective, get a script that has cities, interiors, and vehicles. That will through you into the deep end of the pool. The other part is what you're already doing, which is studying theory. I think Marshall's perspective videos are great. I bought them years ago and they helped me tremendously. Ditto on Scott Robertson's book.

I suggest to draw in the station point. It will help you place the vanishing points and Cone of Vision properly instead of just guessing them. I find it difficult to explain without showing you. If you have Scott Robertson's book "How to Draw" I suggest to look at page 24-25 specifically. Also about that table, the book "Framed Perspective Vol.1" page 56 shows how to construct a table. hope this helps.

Looks like you have the right idea. Your vanishing points seem too close together which is causing distortion, but this is a good way to practice perspective.

Take a picture of a building that's in one point perspective, Then draw it in two, three point perspective. Then draw un up view, then a down shot, etc. This is a good way to gain control of perspective :)

Can you show us some example of your work or studies on perspective? It's easier and more accurate for us to give advice, if you could provide some things you done.

I like that you're emulating the way Proko does gestures. The only thing I want to point out is that in the second pose your drawing is more inclined than the reference, this results in an unbalanced pose. Try to draw a straight line to compare how inclined it should be, the chest is too low and the weight of the arms would make you fall. Another good trick to know how much to exaggerate a pose is by doing the pose yourself. Good luck :)