When adhering to a strict perspective system (one point, two point, three point), you immediately give up a fair bit of flexibility in terms of how the objects you're drawing will be oriented. For example, if everything is drawn according to the same two vanishing points, it will be as though everything was placed on a set grid.
There is a vast advantage to this however - because the orientation of objects becomes restricted, we limit the possible behaviour of every line to only a few options. Many students ignore this fact, however - they lay out their vanishing point(s), then assume they'll know by instinct how every line should behave. Instead of knowing, they guess.
If you ever catch yourself guessing or uncertain about how to draw a line, stop and step back. Take a look at the system you're working with, and think about the short list of possible behaviours. In one point perspective (which we are using for this exercise), you have the following behaviours:
All lines that go off into the distance converge at the vanishing point
All horizontal lines run perfectly parallel to the horizon
All vertical lines run perfectly perpendicular to the horizon
There are simply no other options. Every single line will adhere to one of these three behaviours. So, find which one matches the line you're trying to draw, and then apply it.
It is fair to say that in one point perspective, you can have lines that don't run parallel or perpendicular to the horizon, but - these rules are going to hold fast here however because for the purposes of this exercise, all of our boxes are going to run parallel to the ground plane, rather than being slanted or angled.