gistfile1.swift

class RelativeLayoutConstraint: NSLayoutConstraint {
    
    enum RelativeMetric {
        case None
        case ScreenWidth
        case ScreenHeight
    }
    
    @IBInspectable var percentage: CGFloat = 0
    
    // Unfortuately, enums are not yet supported as IBInspectables. We have to use booleans for each enum case
    /*@IBInspectable*/ var metric: RelativeMetric = .None
    
    @IBInspectable var screenWidth: Bool = false {
        didSet {
            if screenWidth { metric = .ScreenWidth }
        }
    }
    @IBInspectable var screenHeight: Bool = false {
        didSet {
            if screenHeight { metric = .ScreenHeight }
        }
    }
    
    override var constant: CGFloat {
        get {
            if metric != .None {
                let value = maximumValueFor(metric) * percentage / 100.0
                return value
            }
            return super.constant
        }
        set {
            super.constant = newValue
            metric = .None // if 'constant' property is set explicitly, give it the priority against percentage value
        }
    }
    
    private func maximumValueFor(metric: RelativeMetric) -> CGFloat {
        switch metric {
        case .ScreenWidth:
            return UIScreen.mainScreen().bounds.width
        case .ScreenHeight:
            return UIScreen.mainScreen().bounds.height
        case .None:
            return 0
        }
    }
}

You can use this NSLayoutConstraint subclass to specify it's constant to have relative value of screen width or height.