@haidut
Are you able to maintain the 38-40 value without holding your breath? In other words, can you simply inhale and exhale for some time and keep that value constant?
I usually blow 35 when NOT holding my breath and while sitting. After walking until tired or doing some other kind of chores the number gets down to 32. It shows you how quickly physical demands on the body tax the CO2 resources. Maybe I am poor example, as my breathing is not optimal, but I try to breath through my nose all the time.