Complexity is a dial, not a setting
This is the most distinctive thing Campbell teaches about meditation, and almost every secondary account of him drops it.
The mantra is not fixed. Its complexity is something you adjust, continuously, against how much thought traffic you are getting. His instruction is symmetrical and he gives it twice, once for sound and once for images: if intruding thoughts constantly stream into your awareness, give the mantra a more active form. As thoughts disappear and do not return, decrease the complexity.
So a busy mind gets more to chew on — a longer chant, a rotating football, a coat with buttons and seams to find. A quiet mind gets less: the repetition slows, smears, and softens into a background rather than an event.
And there is an exit condition, which he states the same way for each variant. If the sound slips away but no extraneous thoughts appear, let it go and drift in the quiet blankness of your consciousness. His aside on this: you will love it.
The two buttons on the sit screen are that dial. You will not need them constantly. When you notice thinking has taken over, make it busier. When you notice there is nothing much left to push away, make it simpler.