I am playing with Brad Larsen's adaption of the trackball app.
I have two views at a 60 degree angle to each other and was wondering how I get the rotation to be in the center of this (non-closed) rectangle?

In the images below I would have liked the rotation to take place all within the blue lines.

Code (modified to only rotate around x axis):

#import "MyView.h"

//=====================================================
// Defines
//=====================================================

#define DEGREES_TO_RADIANS(degrees) \
    (degrees * (M_PI / 180.0f))

//=====================================================
// Public Interface
//=====================================================

@implementation MyView

- (void)awakeFromNib
{
    transformed = [CALayer layer];
    transformed.anchorPoint = CGPointMake(0.5f, 0.5f);

    transformed.frame = self.bounds;
    [self.layer addSublayer:transformed];

    CALayer *imageLayer = [CALayer layer];
    imageLayer.frame = CGRectMake(10.0f, 4.0f, self.bounds.size.width / 2.0f, self.bounds.size.height / 2.0f);
    imageLayer.transform = CATransform3DMakeRotation(DEGREES_TO_RADIANS(60.0f), 1.0f, 0.0f, 0.0f);
    imageLayer.contents = (id)[[UIImage imageNamed:@"IMG_0051.png"] CGImage];
    imageLayer.borderColor = [UIColor yellowColor].CGColor;
    imageLayer.borderWidth = 2.0f;
    [transformed addSublayer:imageLayer];

    imageLayer = [CALayer layer];
    imageLayer.frame = CGRectMake(10.0f, 120.0f, self.bounds.size.width / 2.0f, self.bounds.size.height / 2.0f);
    imageLayer.transform = CATransform3DMakeRotation(DEGREES_TO_RADIANS(-60.0f), 1.0f, 0.0f, 0.0f);
    imageLayer.contents = (id)[[UIImage imageNamed:@"IMG_0089.png"] CGImage];
    imageLayer.borderColor = [UIColor greenColor].CGColor;
    imageLayer.borderWidth = 2.0f;

    transformed.borderColor = [UIColor whiteColor].CGColor;
    transformed.borderWidth = 2.0f;
    [transformed addSublayer:imageLayer];

    UIView *line = [[UIView alloc] initWithFrame:CGRectMake(0, self.bounds.size.height / 2.0f, self.bounds.size.width, 2)];
    [line setBackgroundColor:[UIColor redColor]];
    [self addSubview:line];

    line = [[UIView alloc] initWithFrame:CGRectMake(0, self.bounds.size.height * (1.0f / 4.0f), self.bounds.size.width, 2)];
    [line setBackgroundColor:[UIColor blueColor]];
    [self addSubview:line];

    line = [[UIView alloc] initWithFrame:CGRectMake(0, self.bounds.size.height * (3.0f / 4.0f), self.bounds.size.width, 2)];
    [line setBackgroundColor:[UIColor blueColor]];
    [self addSubview:line];
}

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    previousLocation = [[touches anyObject] locationInView:self];
}

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
    CGPoint location = [[touches anyObject] locationInView:self];
    //location = CGPointMake(previousLocation.x, location.y);

    CATransform3D currentTransform = transformed.sublayerTransform;

    //CGFloat displacementInX = location.x - previousLocation.x;
    CGFloat displacementInX = previousLocation.x - location.x;
    CGFloat displacementInY = previousLocation.y - location.y;

    CGFloat totalRotation = sqrt((displacementInX * displacementInX) + (displacementInY * displacementInY));

    CGFloat angle = DEGREES_TO_RADIANS(totalRotation);
    CGFloat x = ((displacementInX / totalRotation) * currentTransform.m12 + (displacementInY/totalRotation) * currentTransform.m11);

    CATransform3D rotationalTransform = CATransform3DRotate(currentTransform, angle, x, 0, 0);

    previousLocation = location;
    transformed.sublayerTransform = rotationalTransform;
}

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event {
}

- (void)dealloc {
  [super dealloc];
}

@end