I have 50 Armourer and 50 Blacksmith. I have made over 500k gil making large amounts of Darksteel Nuggets. I buy the Ore needed to make the Nuggets and I make as many Nuggets as I can. My Profitable gain becomes greater and greater every time I do this as I invest all of my batch profits to my next bigger batch. The only issues I have ever had with this method is the notorious Under-Cutters.