If you want the same proportional size as 115 is to 480 (0.24), then you use the multiplier parameter in constraintWithItem:attribute:relatedBy:toItem:attribute:multiplier:constant:.
In the example below, I made an IBOutlet to the height constraint in IB (heightCon), then remove that in code and add another one that is based on a fraction of self.view's height (iv is my outlet to the image view):
- (void)viewDidLoad {
[super viewDidLoad];
[self.iv removeConstraint:self.heightCon];
NSLayoutConstraint *newHeightCon = [NSLayoutConstraint constraintWithItem:self.iv attribute:NSLayoutAttributeHeight relatedBy:0 toItem:self.view attribute:NSLayoutAttributeHeight multiplier:.24 constant:0];
[self.view addConstraint:newHeightCon];
}