As acknowledged, you can't actually run it with a gap else the bikes weight on it in use will push it in further.
You may find a limit in how far it will tighten by the bottom yoke touching the frame first and preventing you going any further.
As you say though, a spacer above will give you the clearance and correct geometry.
One of the advantages of a spacer on the yoke that I've seen, is to incorporate an O-ring on the outside diameter of it that will make an effective dust seal on the inside of the frame's headstock.
When tightening them you can use about 20lbs of torque while gently rotating the bearing to squeeze them into final seating position. When it's ok, release the load and nip it tight again before the locking ring is secured.