Extend the tungsten out of cup 1/4” to 5/16” when welding high amperage aluminum or heat will radiate from the cup to tungsten and reduce output, especially if the orifice is too small.
Have you ever seen small diameter TIG cups glow red from extended welding at high amperage?...me too.
All that heat close to the tungsten electrode will make it get even hotter and reduce the amperage carrying capability
...and there is always an exception to every rule. some welders dont extend the electrode at all when welding aluminum castings to avoid the possibility of tungsten inclusions.