I think there is a different way to interpret the 50% rule, or 33%, whichever it is you want to go with. That "other" should be art or practice that is not related to DrawABox. At the very least, it should be something that interests you, something that breaks up the monotony and helps you focus on a different aspect of drawing. There should be something about your practice that is free from structure, in the sense that structure comes from a course rather than your own direction.

A 33% rule is similar to what I've done so far. In the past month I have spent roughly 33% of my drawing practice on DrawABox, about 33% on a gesture/figure drawing course, and then 33% on other. That 33% "fun" is usually not actually very fun, but I am very much seeing the merits in that it gets me to draw outside of what courses are asking for. The gesture course has also been highly beneficial because it helps with doing fast and loose drawings in contrast with the incredibly structured nature of DrawABox. Just some thoughts to put out there, hopefully that helps though!