Here's what I usually do.
1. First, a yard of material has a little more than 1500 sq in. (36X42=1512 sq")
2. I then find out how many square inces are needed for the block and multiply that by 12 for the 12 blocks.
3. Divide that total by 1500 and that is approx. the amount of yardage you will need. I always round up a bit to account for discrepancies.
Prairie Queen:
Color 1: 1 Yard
Approx. 100 sq" for 1 block. 1200sq" for 12 blocks. 1200/1500=.8 yds. Round up to 1 yard.
Color 2: 1/2 yard
Approx. 45.25 sq" for 1 block. 543 sq" for 12 blocks. 543/1500=.362 yds. Round up to 1/2 yard.
Color 3: 3/4 yards
Approx. 74 sq" for 1 block. 888sq" for 12 blocks. 888/1500=.592 yards. round up to 3/4 yards.
You can always buy a little extra just in case.