Zonk, here's a couple more cents worth from a non-expert. I use the shortest electrode extention that I can get a stable arc with. The shorter the better since it helps to keep contamination away.
Also depending on how tight the joint access is, I will play around with different nozzel diameters. I like to use a small diameter tungsten and nozzel on thin materials.
I don't do much aluminum so when I do I spend more time practicing on scrap before I get to the objective...lol.