Wow...I did a quick search and came up with this site. I am amazed. I wish I would have found this site a while back. It looks like it only comes up with one yardage for one colour, ie for HST's I threw in a block size and the quantity and it came up with 1.5 yards. I guess you would buy 3/4 yard of each colour.
But it's better than guessing blind.
Oh, and the best part.....it tells you how many strips to cut and what size...then, you can print the calculations.