1) Ah ha! Thanks Mark, that is a great explanation. Very succinct about Y and X. Perfect, thank you.
2) Sorry, I should have explained a little more about my application and what I'm trying to find: the IMU stays static and tilts to try to find level. I'm trying to find the tilt angle between the Z axis and the gravity vector, regardless of how much it has rotated around the Z angle. I've attached a diagram (from digikey website) and angle shown in figure (d) is what I'm trying to achieve. My thinking is this should always show the compound X and Y tilt value as a single angle value.
What do you think?