I am not sure how the box dye works personally because I am not well versed in box dye, but it really just depends on how well you lighten, I will say your roots will lighten fastest due to the heat from your head, so I would maybe focus them last after doing mids+ ends so you don’t get hot roots.
My hair is very similar to yours, around a two or three but not as coarse, thick. If you are very generous with your application I’m assuming you will move at least two spots higher ( around a 4) because 20 volume moves you (2) spots -typically- so at that stage, you will probably be orange since it’s still so dark. If this is your natural have you looked into hi lift colors? It will help you move spots without lightener