How they adjust it probably depends on the type of scale events they enter. In FAI (international and world championship events), having the needle valve extension hanging out in the breeze will cost you static points since it doesn't appear on the full scale plane you are modeling. Other than that, I can't answer your question. If you use the tube as a guide for a screwdriver, I would assume that it is rigidly mounted in the airframe so that the screwdriver comes out very close to where it is needed.