Swift 3.0 вы используете как
let widthfraction = 2.0 //Your desired value for me 2.0
splitViewController?.preferredPrimaryColumnWidthFraction = 0.40
let minimumWidth = min((splitViewController?.view.bounds.size.width)!,(splitViewController?.view.bounds.height)!)
splitViewController?.minimumPrimaryColumnWidth = minimumWidth / widthFraction
splitViewController?.maximumPrimaryColumnWidth = minimumWidth / widthFraction
let leftNavController = splitViewController?.viewControllers.first as! UINavigationController
leftNavController.view.frame = CGRect(x: leftNavController.view.frame.origin.x, y: leftNavController.view.frame.origin.y, width: (minimumWidth / widthFraction), height: leftNavController.view.frame.height)