The way I look at these things, I feel that anything that is done strictly from a single piece of reference, with the intent of replicating that reference, is a study (and therefore belongs in the same 50% as the rest of the Drawabox work, as an exercise done to learn/improve).

You can however use multiple pieces of reference to help as you draw something of your own ("from your imagination"), and that would count towards the 50% of drawing just for the sake of drawing.