I figure their fabric requirements are just basic amounts and since I normally go thru my fabric stash I know I can alter using 2 or more fabrics for their one. Plus I seem to take their pattern into EQ8 and draw it up using the fabric I've chosen from my stash to see if it looks okay. The program also gives me a round-about figure for fabrics too.
I rarely buy fabrics for a projects, more like I buy fabric cause it talked to me and then find a pattern to fit the amount of fabric I have.