Hi Oliput,
It's a nice start, I suggest you to try other views, to move your vanishing(s) point(s), to explore the cube all around. The goal while working on perspective, is to have an idea of the 3dimensions of it, so reproduce the same cube in different angles to practice.
And also, try to practice in traditionnal, it's harder but you will learn faster for the basics of perspective.
Keep going 
