I have two different dip tubes that I screw onto the bulkhead fitting.  The one in the middle picks up the wort very close to the wall of my converted keg.  This one leaves about 3 quarts in the kettle.  The one on the left draws the wort a little further away and below the first one.  This one only leaves about a quart but tends to draw up some trub, especially when I do hoppy beers.

When I install one of these I stick the valve/bulkhead assembly in the hole but don't tighten down the nut.  I then put on the dip tube fitting by rotating the ball valve around  until the dip tube is on tight.  Then I tighten down the nut once I get the dip tube in the right position.  Before soldering the 90° angle fitting into place during initial fabrication, I dry fit the assembly to the keg so I could mark the correct positioning of the fitting so that the end of the tube will be touching the right place in the keg once the fitting is screwed on tight and the ball valve is in the correct position.
I minimize trub pickup by sticking a small stainless steel scrubbie on the end of the dip tube as I am  installing it for a brew session.  I  always  use at least some, if not all, whole  hops in my brews so this helps with filtration.
Wayne