It comes down to two things, both of which you've mentioned:

• Practice (the obvious one)

• Being aware of the problem and consciously trying to address it as you plot out your lines. One thing students sometimes do is focus too much on the mark they're drawing, to the detriment of the other 3 edges of the set that they're also supposed to be considering. Sometimes they'll pay attention to one other edge, but we have to do our best to consider all four and how they're behaving (including - although it is not easy - those we haven't yet drawn).

There's a lot to the second point, so you may be doing it all, or perhaps you may not be giving yourself enough time to really consider those other edges of a set fully - so as a good starting point, giving yourself more time for the "resolving your corners" step is a good start.

I found it was helpful to first identify what is parallel and then adjust inward from there. Judging relative angles is a skill that has to be built up.

Every convergence is a new angle for every edge of the box but practicing judging parallel is a clear goal that applies to all boxes - even if only in the planning stages. Meaning it is the more beneficial aspect to practice.

But yeah, getting the convergence right is a struggle sometimes. I've seen a lot of very experienced people mess it up as well. Realistically, it only has to be just good enough, not perfect.