Box pleats are basically knife pleats where every other pleat goes the opposite direction.
Think about it this way:
A and B should be ideally the same width--the width of one pleat, half the width of the full "box" for the box pleat. If you want the inside of the box to be wider, you can do A + B + extra width.
Does this help? Try it out on scraps before you work with your main fabric. That's honestly the best way to make sure you get your technique down. Also, make sure you hem your fabric before you pleat it.