I would like to be able to make the menu justified, that covers the whole width of my container and it will be reduced until it shows the mobile menu.
At the moment I have added spacing between the elements but this causes the menu to display in two lines when the window is minimized a little, as shown in the link below.
Then in Customizer > Layout > Primary Navigation try reducing the Menu Item Width on Desktop. The value in there is the left and right padding on the menu items. Reducing that should stop them from breaking to new lines.