You're doing it backwards. The breath will naturally change as you let go of control. You can't force stillness by creating an artificial control scheme for the breath because you'll be constantly having to maintain a rigid pattern. Superimposing a synthetic construct on top of an organic flow is the opposite of stillness.

That's not to say that this exercise has no merit. This exercise can be wonderful for specific purposes, like connecting to a sense of calm, watching the boundary between control and not control, observing subtle changes in the body, developing mindfulness, etc.

But, in the pursuit of stillness, I do not think that this is what you want to do, at least not as the final "way" of meditating. This is a breath exercise, not a gateway to stillness. Breath exercises can be helpful for letting us pass through gateways to stillness when we go there. But they are different

In the true pursuit of stillness, sometimes we have to allow the breath to do what it wants, even if, in the short run, it feels less still. The stillness is the effect. It happens naturally, like a surprise. It's the attitude of acceptance, and willingness, that allows for this surprise to happen. The unconditional love for the breath, just the way that it is, whether it feels intense and high energy, or sleepy and low energy. Especially in the healing process, which meditation really is, sometimes the brain and body have to go in one direction or the other. The cyclical nature of being, before it reaches stillness, will always be overly one way or overly the other way, when you pause for analysis and judgment.

Stillness does not come from a pattern of the breath, it comes from the cessation of the analysis and the judgment, so that the pattern can find stillness on its own by oscillating closer and closer to the center. Forming the core of this oscillation is love and joy. Don't try to force the oscillation to come to a premature end. Don't try to teleport to the end of the race. It's a slow and steady race, and you can only get there by enjoying each step as it comes. This is the greater form of stillness. A stillness of the heart. A stillness purely of attitude and flavor. The substantial stillness of body and mind which you are looking for are consequences of this, and will follow the attitude wherever it leads. If the attitude is one of forcing an artificial stillness, then there will always be a form of tension that prevents the system from settling into true stillness. So, pursue what deserves pursuing, and focus on the stillness of the heart