Best method to resize UIView when rotated to landscape and back

Since I am very new to ios programming I have more of a general-design question. I have a ViewController which contains a GraphView (UIScrollView + UIView) which works fine. When I am rotating to landscape I want the GraphView to resize its height to the display height (so it fills the whole screen) but only 300pts when in portrait.

What I did so far is implementing viewWillLayoutSubviews in the ViewController and resetting the constraints:

- (void)viewWillLayoutSubviews{        
_graphViewHeightConstraint.constant = ([[UIDevice currentDevice] orientation] == UIDeviceOrientationPortrait) ? 300:[[UIScreen mainScreen] bounds].size.height-self.navigationController.navigationBar.frame.size.height - 2*_distanceToTopView.constant; 
}

and in GraphView.m:

- (void)layoutSubviews{
kGraphHeight = self.frame.size.height;
[self setNeedsDisplay];  
}

(because I need the variable kGraphHeight in the code to draw the Graph). This does not seem like a very elegant solution so I wanted to ask what the better way would be? Many thanks for your inputs :)