Using iPhone's accelerometer to move a sprite along the x-axis

I am working on an iPhone game using cocos2d. I am trying to use an accelerometer to move a sprite in portrait mode to move left and right. For some reason the code I use by default moves to the right UNLESS the phone is tilted at a 45 degree angle i.e. The return values ​​are positive (meaning it must move to the right) until it is tilted at a 45 degree angle, either left or right. The dead center comes back around 600 and then tilts the phone down further to the left or right until it reaches a 45-degree angle (where it reaches 0 and starts negative). Below is the code I am using. Any help would be GREAT appraisal.

- (void)accelerometer:(UIAccelerometer *)accelerometer didAccelerate:(UIAcceleration *)acceleration {

        #define kFilteringFactor 0.75
            static UIAccelerationValue rollingX = 0, rollingY = 0, rollingZ = 0;

            rollingX = (acceleration.x * kFilteringFactor) +
            (rollingX * (1.0 - kFilteringFactor));
            rollingY = (acceleration.y * kFilteringFactor) +
            (rollingY * (1.0 - kFilteringFactor));
            rollingZ = (acceleration.z * kFilteringFactor) +
            (rollingZ * (1.0 - kFilteringFactor));

            float accelX = rollingX;
            float accelY = rollingY;
            float accelZ = rollingZ;

            CGSize winSize = [CCDirector sharedDirector].winSize;

        #define kRestAccelX 0.6
        #define kShipMaxPointsPerSec (winSize.height*0.5)
        #define kMaxDiffX 0.2

            float accelDiffX = kRestAccelX - ABS(accelX);
            float accelFractionX = accelDiffX / kMaxDiffX;
            float pointsPerSecX = kShipMaxPointsPerSec * accelFractionX;

            _shipPointsPerSecX = pointsPerSecX;
            NSLog(@"_shipPointsPerSecX: %f", _shipPointsPerSecX);
}

- (void)updateShipPos:(ccTime)dt {

    CGSize winSize = [CCDirector sharedDirector].winSize;

    float maxX = winSize.width - _ship.contentSize.width/2;
    float minX = _ship.contentSize.width/2;

    float newX = _ship.position.x + (_shipPointsPerSecX * dt);
    newX = MIN(MAX(newX, minX), maxX);
    _ship.position = ccp(newX, _ship.position.y);
   // NSLog(@"newx: %f", newX);

}