At the last guild meeting I went to I sat next to a lady that was working on cathedral windows blocks. She said she always does the initial seams across the ends on the machine because it makes the final blocks more accurate. She said it takes between 30 and 40 yards of muslin for a queen size quilt. With today's deeper mattresses I'd just get the full 50 yard bolt in case you need to make it a little bigger. Even if you don't use all of it for the CW you'll find uses for any muslin you have left.