I'm a drafter by trade (although I don't do that work any longer) but to be honest, my figuring out how much fabric is required usually gets used ONLY when I have a pattern in mind and the fabric already at the house. The "I wonder if this is enough" syndrome. My fabric shopping experience is usually - Oh heck give me 2 yards, no make that 3, oh just give me what's left on the bolt..... (in my dreams) ROFLOL
DH learned long time ago that purchase of fabric does not have to correlate to project in mind.