The link to the strawing formula is:
http://forum.lugerforum.com/showthread.php?t=20598
I am interested in trying the chemical strawing on a 1916 DWM shooter, matching numbers but heavily polished and reblued, including the originally strawed parts. I think a surface chemical treatment reduces the risk of harming these old parts, especially the ejector, with heating by a novice. Also, I believe the chemical coloring is more durable.