Android accelerometer angle calculation

I think X should be 0 and y should be positive and z close to 0 for a compass pointing down to the ground. (This means the phone is held vertically).

In general, from angle 0, the compass angle should be something like

float accelerometerMaxRange = 10; // This is NOT right, but it a good value to work with
float newAngle = 0;
if (z > 9) {
    // Phone is horizontally flat, can't point towards gravity, really. Do whatever you think is right
} else {
    newAngle  = (float)(x * 90 / accelerometerMaxRange);
    if (y < 0) {
        newAngle = 180 - newAngle;
    }
}