My long armer says she prefers nothing over 400. I usually buy my sheets for quilt backs at target. They sell the flat sheets separately and they are so soft. They are pretty inexpensive to start and sometimes you can find them on sale, also they come in several colors. I have also never had a problem with these sheets being big enough. A queen sheet fits a queen quilt. I have had problems with other sheets being too small.
http://www.target.com/p/threshold-ul...erm=flat+sheet