Breathing does 2 things, provides oxygen for the cells in our bodies and removes carbon dioxide waste.
Our bodies are adaptive and naturally breathe at the optimal rate to continue these 2 essential functions in different situations.
If you are slowing down your breathing by force to the point where these aren't happening you are both starving your cells of oxygen which they need to live and poisoning them with carbon dioxide. I'm not a scientist so I don't know the tolerances, but that doesn't sound good to me.
I get that by slowing and controlling your breathing it can be relaxing and give you greater control of your thought processes, I use it too, but there's a balance in everything with our bodies and we need to always be aware of that. It's not really something we need to quantify with numbers, we can just feel it if we are receptive to the messages.
The nearest we ever come to knowing truth is when we are witness to paradox.