I'm not sure if this will work, but couldn't you create three global variables:
and on start of layout set startAlpha to Touch.Alpha etcetera?
you could also choose to make a button which the player could touch on the moment he or she wants the phone/tablet to be considered straight and set the values on touch.
after that you should use touch.alpha-startAlpha to get the correct alpha value..
I would try it this way, but if this works?