If you want the height of your label relative to the height of your cell and you use Storyboard, you should determine the size of your label with constraints in relation to the ContentView of the cell. Just set the top and bottom distance from your label to the ContentView and no additional height for the label.
If you dont know how to set Storyboard and constraints, this might help: https://developer.apple.com/library/ios/documentation/UserExperience/Conceptual/AutolayoutPG/AutoLayoutbyExample/AutoLayoutbyExample.html