Shamataha is not about breathing techniques, it is a meditational technique concerned with awareness of the breath "as it is".
It is NOT about guiding or manipulating the breath. It is about being aware of your breath. Initially aware of the inhalation and exhalation OR the quality of the breath, ie if the inhalation is shallow or deep (but note that even with this meditation one is merely aware of the quality of the breath, one does not try to manipulate the breath, or get into analysis like: "this inhalation is shallow, should it be shallow? maybe it should be deep? I know, I will make the next one deep. yes that's it, now I am breathing correctly! ad nauseum...) OR the contact the breath makes at the tip of the nose as it enters or exits.
Shamatha is about awareness, it just happens to take the breath as an object with which to practice awareness.
The breath will do what it needs to do, don't worry about that!
As for the back pain, sit on a chair for longer meditation periods. For the shorter ones use the "correct" posture, with time you will find it comfortable. Until that itme comes though you still need to practice with your mind!
In closing, if you want to do breathing techniques find yourself a good teacher. Over-exertion during breathing techniques can actually damage the lungs, seeing how you already have a problem with your lungs you don't want to cause any more!