Perfacar Got it right,though it is old school the paint burn trick was how we tuned them back in the day.Most start with 18'' to 20'' lightly paint a stripe, make a pass and see where the paint burned off. Rule of thumb was, stick car cut the burn off, automatic cut at 1'' longer than the burn.You have yours completely painted thats ok too as it will burn a ring around the tube just as well.you may have to make more than 1 pass.
This is really old school but does work.Does "Hooker Adjustable Headers" ring a bell with anyone ?