For something with a simple, single seam, like I imagine the windshirt sleeve is, then I'd simply lay the garment out flat so that the arm seam is one of the folded edges, and then trace around the outline of the arm, and the two sides of the armscye. The arm seams are usually simple straight lines. The scyes are curved... and front and back are usually different (depending on the scye design).
Then add the seam allowance, depending on the type of seam you're planning to use.
I've used this method for other items, such as fleeces, with Raglan sleeves, and multi-piece sleeves. It worked fine for me.
 d'oh! you've sorted it...